acinclude.m4: make kernel-specific flags cacheable

Specifically when cross-compiling, one can be willing to force these
variable values using the environment of a config.cache file. This
commit makes this possible.

The affected variables are:

* libzmq_cv_sock_cloexec
* libzmq_cv_so_keepalive
* libzmq_cv_tcp_keepcnt
* libzmq_cv_tcp_keepidle
* libzmq_cv_tcp_keepintvl
* libzmq_cv_tcp_keepalive
Signed-off-by: Lionel Orry <lionel.orry@gmail.com>

acinclude.m4

@@ -586,8 +586,8 @@ dnl # LIBZMQ_CHECK_SOCK_CLOEXEC([action-if-found], [action-if-not-found])
 dnl # Check if SOCK_CLOEXEC is supported                                           #
 dnl ################################################################################
 AC_DEFUN([LIBZMQ_CHECK_SOCK_CLOEXEC], [{
-    AC_MSG_CHECKING(whether SOCK_CLOEXEC is supported)
-    AC_TRY_RUN([/* SOCK_CLOEXEC test */
+    AC_CACHE_CHECK([whether SOCK_CLOEXEC is supported], [libzmq_cv_sock_cloexec],
+        [AC_TRY_RUN([/* SOCK_CLOEXEC test */
 #include <sys/types.h>
 #include <sys/socket.h>
 
@@ -597,10 +597,12 @@ int main (int argc, char *argv [])
     return (s == -1);
 }
         ],
-    [AC_MSG_RESULT(yes) ; libzmq_cv_sock_cloexec="yes" ; $1],
-    [AC_MSG_RESULT(no)  ; libzmq_cv_sock_cloexec="no"  ; $2],
-    [AC_MSG_RESULT(not during cross-compile) ; libzmq_cv_sock_cloexec="no"]
+        [libzmq_cv_sock_cloexec="yes"],
+        [libzmq_cv_sock_cloexec="no"],
+        [libzmq_cv_sock_cloexec="not during cross-compile"]
+        )]
     )
+    AS_IF([test "x$libzmq_cv_sock_cloexec" = "xyes"], [$1], [$2])
 }])
 
 dnl ################################################################################
@@ -628,8 +630,8 @@ dnl # LIBZMQ_CHECK_SO_KEEPALIVE([action-if-found], [action-if-not-found])
 dnl # Check if SO_KEEPALIVE is supported                                           #
 dnl ################################################################################
 AC_DEFUN([LIBZMQ_CHECK_SO_KEEPALIVE], [{
-    AC_MSG_CHECKING(whether SO_KEEPALIVE is supported)
-    AC_TRY_RUN([/* SO_KEEPALIVE test */
+    AC_CACHE_CHECK([whether SO_KEEPALIVE is supported], [libzmq_cv_so_keepalive],
+        [AC_TRY_RUN([/* SO_KEEPALIVE test */
 #include <sys/types.h>
 #include <sys/socket.h>
 
@@ -642,10 +644,12 @@ int main (int argc, char *argv [])
     );
 }
         ],
-    [AC_MSG_RESULT(yes) ; libzmq_cv_so_keepalive="yes" ; $1],
-    [AC_MSG_RESULT(no)  ; libzmq_cv_so_keepalive="no"  ; $2],
-    [AC_MSG_RESULT(not during cross-compile) ; libzmq_cv_so_keepalive="no"]
+        [libzmq_cv_so_keepalive="yes"],
+        [libzmq_cv_so_keepalive="no"],
+        [libzmq_cv_so_keepalive="not during cross-compile"]
+        )]
     )
+    AS_IF([test "x$libzmq_cv_so_keepalive" = "xyes"], [$1], [$2])
 }])
 
 dnl ################################################################################
@@ -653,8 +657,8 @@ dnl # LIBZMQ_CHECK_TCP_KEEPCNT([action-if-found], [action-if-not-found])
 dnl # Check if TCP_KEEPCNT is supported                                            #
 dnl ################################################################################
 AC_DEFUN([LIBZMQ_CHECK_TCP_KEEPCNT], [{
-    AC_MSG_CHECKING(whether TCP_KEEPCNT is supported)
-    AC_TRY_RUN([/* TCP_KEEPCNT test */
+    AC_CACHE_CHECK([whether TCP_KEEPCNT is supported], [libzmq_cv_tcp_keepcnt],
+        [AC_TRY_RUN([/* TCP_KEEPCNT test */
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -670,10 +674,12 @@ int main (int argc, char *argv [])
     );
 }
         ],
-    [AC_MSG_RESULT(yes) ; libzmq_cv_tcp_keepcnt="yes" ; $1],
-    [AC_MSG_RESULT(no)  ; libzmq_cv_tcp_keepcnt="no"  ; $2],
-    [AC_MSG_RESULT(not during cross-compile) ; libzmq_cv_tcp_keepcnt="no"]
+        [libzmq_cv_tcp_keepcnt="yes"],
+        [libzmq_cv_tcp_keepcnt="no"],
+        [libzmq_cv_tcp_keepcnt="not during cross-compile"]
+        )]
     )
+    AS_IF([test "x$libzmq_cv_tcp_keepcnt" = "xyes"], [$1], [$2])
 }])
 
 dnl ################################################################################
@@ -681,8 +687,8 @@ dnl # LIBZMQ_CHECK_TCP_KEEPIDLE([action-if-found], [action-if-not-found])
 dnl # Check if TCP_KEEPIDLE is supported                                           #
 dnl ################################################################################
 AC_DEFUN([LIBZMQ_CHECK_TCP_KEEPIDLE], [{
-    AC_MSG_CHECKING(whether TCP_KEEPIDLE is supported)
-    AC_TRY_RUN([/* TCP_KEEPIDLE test */
+    AC_CACHE_CHECK([whether TCP_KEEPIDLE is supported], [libzmq_cv_tcp_keepidle],
+        [AC_TRY_RUN([/* TCP_KEEPIDLE test */
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -698,10 +704,12 @@ int main (int argc, char *argv [])
     );
 }
         ],
-    [AC_MSG_RESULT(yes) ; libzmq_cv_tcp_keepidle="yes" ; $1],
-    [AC_MSG_RESULT(no)  ; libzmq_cv_tcp_keepidle="no"  ; $2],
-    [AC_MSG_RESULT(not during cross-compile) ; libzmq_cv_tcp_keepidle="no"]
+        [libzmq_cv_tcp_keepidle="yes"],
+        [libzmq_cv_tcp_keepidle="no"],
+        [libzmq_cv_tcp_keepidle="not during cross-compile"]
+        )]
     )
+    AS_IF([test "x$libzmq_cv_tcp_keepidle" = "xyes"], [$1], [$2])
 }])
 
 dnl ################################################################################
@@ -709,8 +717,8 @@ dnl # LIBZMQ_CHECK_TCP_KEEPINTVL([action-if-found], [action-if-not-found])
 dnl # Check if TCP_KEEPINTVL is supported                                           #
 dnl ################################################################################
 AC_DEFUN([LIBZMQ_CHECK_TCP_KEEPINTVL], [{
-    AC_MSG_CHECKING(whether TCP_KEEPINTVL is supported)
-    AC_TRY_RUN([/* TCP_KEEPINTVL test */
+    AC_CACHE_CHECK([whether TCP_KEEPINTVL is supported], [libzmq_cv_tcp_keepintvl],
+        [AC_TRY_RUN([/* TCP_KEEPINTVL test */
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -726,10 +734,12 @@ int main (int argc, char *argv [])
     );
 }
         ],
-    [AC_MSG_RESULT(yes) ; libzmq_cv_tcp_keepintvl="yes" ; $1],
-    [AC_MSG_RESULT(no)  ; libzmq_cv_tcp_keepintvl="no"  ; $2],
-    [AC_MSG_RESULT(not during cross-compile) ; libzmq_cv_tcp_keepintvl="no"]
+        [libzmq_cv_tcp_keepintvl="yes"],
+        [libzmq_cv_tcp_keepintvl="no"],
+        [libzmq_cv_tcp_keepintvl="not during cross-compile"]
+        )]
     )
+    AS_IF([test "x$libzmq_cv_tcp_keepintvl" = "xyes"], [$1], [$2])
 }])
 
 dnl ################################################################################
@@ -737,8 +747,8 @@ dnl # LIBZMQ_CHECK_TCP_KEEPALIVE([action-if-found], [action-if-not-found])
 dnl # Check if TCP_KEEPALIVE is supported                                          #
 dnl ################################################################################
 AC_DEFUN([LIBZMQ_CHECK_TCP_KEEPALIVE], [{
-    AC_MSG_CHECKING(whether TCP_KEEPALIVE is supported)
-    AC_TRY_RUN([/* TCP_KEEPALIVE test */
+    AC_CACHE_CHECK([whether TCP_KEEPALIVE is supported], [libzmq_cv_tcp_keepalive],
+        [AC_TRY_RUN([/* TCP_KEEPALIVE test */
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -754,10 +764,12 @@ int main (int argc, char *argv [])
     );
 }
         ],
-    [AC_MSG_RESULT(yes) ; libzmq_cv_tcp_keepalive="yes" ; $1],
-    [AC_MSG_RESULT(no)  ; libzmq_cv_tcp_keepalive="no"  ; $2],
-    [AC_MSG_RESULT(not during cross-compile) ; libzmq_cv_tcp_keepalive="no"]
+        [libzmq_cv_tcp_keepalive="yes"],
+        [libzmq_cv_tcp_keepalive="no"],
+        [libzmq_cv_tcp_keepalive="not during cross-compile"]
+        )]
     )
+    AS_IF([test "x$libzmq_cv_tcp_keepalive" = "xyes"], [$1], [$2])
 }])
 
 dnl ################################################################################