Check usability of inotify before selecting it

A libinotify compatibility library exists for BSD systems. If this
was installed, configure would find sys/inotify.h but the build
would fail to link unless the library was added to the link line.
By also checking for inotify_init() this problem can be avoided.

Given that there is native support for kqueue, there is no need
to handle linking with libinotify.

configure.in

@@ -5476,10 +5476,17 @@ if test "$wxUSE_FSWATCHER" = "yes"; then
     dnl includes OS X which does have kqueue but no other platforms)
     if test "$USE_WIN32" != 1; then
         if test "$wxUSE_UNIX" = "yes"; then
-            AC_CHECK_HEADERS(sys/inotify.h,,, [AC_INCLUDES_DEFAULT()])
-            if test "$ac_cv_header_sys_inotify_h" = "yes"; then
-                AC_DEFINE(wxHAS_INOTIFY)
-            else
+            dnl inotify header may be present from a compatibility library so
+            dnl check that the function is usable. We don't try to link with,
+            dnl e.g. -linotify so native kqueue support is used in preference.
+            AC_MSG_CHECKING([whether inotify is usable])
+            AC_LINK_IFELSE(
+                [AC_LANG_SOURCE([int main() { return inotify_init(); }])],
+                [wx_cv_inotify_usable=yes; AC_DEFINE(wxHAS_INOTIFY) ],
+                [wx_cv_inotify_usable=no]
+            )
+            AC_MSG_RESULT($wx_cv_inotify_usable)
+            if test "$wx_cv_inotify_usable" = "no"; then
                 AC_CHECK_HEADERS(sys/event.h,,, [AC_INCLUDES_DEFAULT()])
                 if test "$ac_cv_header_sys_event_h" = "yes"; then
                     AC_DEFINE(wxHAS_KQUEUE)