autotools: tests: disable test_fork if fork() is not available

fork() support is optional and its availability is correctly detected at
contfigure time.

But test_fork was all always built, preventing build for targets that do
not provide fork() from building successfully.

This pacth fixes the autotools on this point.

configure.ac

@@ -544,6 +544,8 @@ LIBZMQ_CHECK_TCP_KEEPALIVE([AC_DEFINE(
                               [Whether TCP_KEEPALIVE is supported.])
                           ])
 
+AM_CONDITIONAL(HAVE_FORK, test "x$ac_cv_func_fork" = "xyes")
+
 # Subst LIBZMQ_EXTRA_CFLAGS & CXXFLAGS & LDFLAGS
 AC_SUBST(LIBZMQ_EXTRA_CFLAGS)
 AC_SUBST(LIBZMQ_EXTRA_CXXFLAGS)

tests/Makefile.am

@@ -56,8 +56,10 @@ noinst_PROGRAMS += test_shutdown_stress \
                    test_pair_ipc \
                    test_reqrep_ipc \
                    test_timeo \
-                   test_fork \
-                   test_filter_ipc 
+                   test_filter_ipc
+if HAVE_FORK
+noinst_PROGRAMS += test_fork
+endif
 endif
 
 if BUILD_TIPC
@@ -123,7 +125,9 @@ test_shutdown_stress_SOURCES = test_shutdown_stress.cpp
 test_pair_ipc_SOURCES = test_pair_ipc.cpp testutil.hpp
 test_reqrep_ipc_SOURCES = test_reqrep_ipc.cpp testutil.hpp
 test_timeo_SOURCES = test_timeo.cpp
+if HAVE_FORK
 test_fork_SOURCES = test_fork.cpp
+endif
 test_filter_ipc_SOURCES = test_filter_ipc.cpp
 endif
 if BUILD_TIPC