fallback on tweetnacl if libsodium is not found and not explicitly requested

configure.ac

@@ -427,19 +427,34 @@ fi
 have_sodium_library="no"
 
 AC_ARG_WITH([libsodium], [AS_HELP_STRING([--with-libsodium],
-    [require libzmq build with libsodium crypto library. Requires pkg-config [default=yes]])],
+    [require libzmq build with libsodium crypto library. Requires pkg-config [default=check]])],
     [require_libsodium_ext=$withval],
-    [require_libsodium_ext=yes])
+    [require_libsodium_ext=check])
 
 AC_ARG_WITH([tweetnacl], [AS_HELP_STRING([--with-tweetnacl],
     [build libzmq  with bundled tweetnacl crypto library [default=no]])],
     [require_libsodium_ext=no
-     with_tweetnacl=yes],
-    [with_tweetnacl=no])
+     with_tweetnacl=yes
+     AC_MSG_CHECKING(for sodium)
+     AC_MSG_RESULT(tweetnacl)],
+    [with_tweetnacl=check])
 
 # conditionally require libsodium package
 if test "x$require_libsodium_ext" != "xno"; then
-    PKG_CHECK_MODULES([sodium], [libsodium], [have_sodium_library="yes"])
+  PKG_CHECK_MODULES([sodium], [libsodium],
+    [
+      have_sodium_library=yes
+      with_tweetnacl=no
+    ],
+    [
+      if test "x$require_libsodium_ext" == "xyes"; then
+        AC_MSG_ERROR(libsodium has been requested but not found)
+      else
+        AC_MSG_RESULT([             libsodium not found, using tweetnacl])
+        have_sodium_library=no
+        with_tweetnacl=yes
+      fi
+    ])
 fi
 
 if test "x$have_sodium_library" != "xno"; then
@@ -453,7 +468,6 @@ if test "x$have_sodium_library" != "xno"; then
         ;;
     esac
 elif test "x$with_tweetnacl" != "xno"; then
-    echo "using tweetnacl"
     AC_DEFINE(HAVE_LIBSODIUM, 1, [Sodium is provided by tweetnacl.])
     AC_DEFINE(HAVE_TWEETNACL, 1, [Using tweetnacl.])
 fi