configure: add detection for O_NOFOLLOW flag to the open function

As discovered by Douglas Torrance, this flag is not really portable, so
this patch adds a check in the configure script to detect if any known
define could help. If no value works, we fall back to defining it with the
neutral value '0', so the compilation will not fail.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

m4/windowmaker.m4

@@ -207,3 +207,40 @@ AS_IF([test "x$wm_cv_func_secure_getenv" != "xno"],
      AC_DEFINE([HAVE_SECURE_GETENV], [1],
          [defined when GNU's secure_getenv function is available])])
 ])
+
+
+# WM_FUNC_OPEN_NOFOLLOW
+# ---------------------
+#
+# Check if the flag 'O_NOFOLLOW' is supported, for the function 'open'
+AC_DEFUN_ONCE([WM_FUNC_OPEN_NOFOLLOW],
+[AC_CACHE_CHECK([for O_NOFOLLOW], [wm_cv_func_open_nofollow],
+    [wm_cv_func_open_nofollow=no
+     wm_save_CPPFLAGS="$CPPFLAGS"
+     for wm_arg in dnl
+"yes"  dnl natively supported, nothing to do
+"-D_POSIX_C_SOURCE=200809L"  dnl the flag was officially added in POSIX.1-2008
+"-D_XOPEN_SOURCE=700"  dnl for recent glibc
+"-D_GNU_SOURCE"  dnl for older glibc
+     ; do
+         AS_IF([test "x$wm_arg" != "xyes"], [CPPFLAGS="$wm_save_CPPFLAGS $wm_arg"])
+         AC_LINK_IFELSE([AC_LANG_PROGRAM([dnl
+@%:@include <sys/types.h>
+@%:@include <sys/stat.h>
+@%:@include <fcntl.h>], [dnl
+  int fd;
+
+  fd = open("/dev/null", O_RDONLY | O_NOFOLLOW);
+  return fd;])], [found=1], [found=0])
+         AS_IF([test $found = 1],
+             [wm_cv_func_open_nofollow="$wm_arg"
+              break])
+     done
+     CPPFLAGS="$wm_save_CPPFLAGS"])
+AS_CASE([$wm_cv_func_open_nofollow],
+    [yes], [],
+    [no],  [AC_DEFINE([O_NOFOLLOW], [0],
+                [defined by configure if the attribute is not defined on your platform])
+            AC_MSG_WARN([flag O_NOFOLLOW is not defined on your platform])],
+    [CPPFLAGS="$CPPFLAGS $wm_cv_func_open_nofollow"])
+])