Add a compile-time check for FORTIFY_SOURCE support

This is broken on some platforms, so pull in the conditional macro from
the latest version of autoconf-archive to test for working support.

m4/ax_add_fortify_source.m4

@@ -0,0 +1,80 @@
+# ===========================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_add_fortify_source.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_ADD_FORTIFY_SOURCE
+#
+# DESCRIPTION
+#
+#   Check whether -D_FORTIFY_SOURCE=2 can be added to CPPFLAGS without macro
+#   redefinition warnings, other cpp warnings or linker. Some distributions
+#   (such as Gentoo Linux) enable _FORTIFY_SOURCE globally in their
+#   compilers, leading to unnecessary warnings in the form of
+#
+#     <command-line>:0:0: error: "_FORTIFY_SOURCE" redefined [-Werror]
+#     <built-in>: note: this is the location of the previous definition
+#
+#   which is a problem if -Werror is enabled. This macro checks whether
+#   _FORTIFY_SOURCE is already defined, and if not, adds -D_FORTIFY_SOURCE=2
+#   to CPPFLAGS.
+#
+#   Newer mingw-w64 msys2 package comes with a bug in
+#   headers-git-7.0.0.5546.d200317d-1. It broke -D_FORTIFY_SOURCE support,
+#   and would need -lssp or -fstack-protector.  See
+#   https://github.com/msys2/MINGW-packages/issues/5803. Try to actually
+#   link it.
+#
+# LICENSE
+#
+#   Copyright (c) 2017 David Seifert <soap@gentoo.org>
+#   Copyright (c) 2019 Reini Urban <rurban@cpan.org>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 4
+
+AC_DEFUN([AX_ADD_FORTIFY_SOURCE],[
+    ac_save_cflags=$CFLAGS
+    ac_cwerror_flag=yes
+    AX_CHECK_COMPILE_FLAG([-Werror],[CFLAGS="$CFLAGS -Werror"])
+    AC_MSG_CHECKING([whether to add -D_FORTIFY_SOURCE=2 to CPPFLAGS])
+    AC_LINK_IFELSE([
+        AC_LANG_PROGRAM([],
+            [[
+                #ifndef _FORTIFY_SOURCE
+                    return 0;
+                #else
+                    this_is_an_error;
+                #endif
+            ]]
+        )],
+        AC_LINK_IFELSE([
+            AC_LANG_SOURCE([[
+                #define _FORTIFY_SOURCE 2
+                #include <string.h>
+                int main() {
+                    char *s = " ";
+                    strcpy(s, "x");
+                    return strlen(s)-1;
+                }
+              ]]
+            )],
+            [
+              AC_MSG_RESULT([yes])
+              CFLAGS=$ac_save_cflags
+              CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"
+            ], [
+              AC_MSG_RESULT([no])
+              CFLAGS=$ac_save_cflags
+            ],
+        ),
+        [
+          AC_MSG_RESULT([no])
+          CFLAGS=$ac_save_cflags
+        ])
+])

m4/check-hardening-options.m4

@@ -73,7 +73,7 @@ AC_DEFUN([CHECK_C_HARDENING_OPTIONS], [
 		CHECK_CFLAG([[-fno-strict-overflow]])
 
 		# _FORTIFY_SOURCE replaces builtin functions with safer versions.
-		CHECK_CFLAG([[-D_FORTIFY_SOURCE=2]])
+		AX_ADD_FORTIFY_SOURCE
 
 		# Enable read only relocations
 		CHECK_LDFLAG([[-Wl,-z,relro]])