Adding -D_FORTIFY_SOURCE=2 when possible

configure.ac

@@ -853,6 +853,10 @@ AS_IF([test "$enable_stack_protector" = "yes"],
     ])
  ])
 
+AS_IF([test x$ac_cv_c_compiler_gnu = xyes],
+    [AX_ADD_FORTIFY_SOURCE]
+)
+
 CFLAGS="$CFLAGS -W"
 
 warn_CFLAGS="-Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes"

m4/ax_add_fortify_source.m4

@@ -0,0 +1,53 @@
+# ===========================================================================
+#   Modified from 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 CFLAGS without macro
+#   redefinition warnings. 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 CFLAGS.
+#
+# LICENSE
+#
+#   Copyright (c) 2017 David Seifert <soap@gentoo.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 1
+
+AC_DEFUN([AX_ADD_FORTIFY_SOURCE],[
+    AC_MSG_CHECKING([whether to add -D_FORTIFY_SOURCE=2 to CFLAGS])
+    AC_LINK_IFELSE([
+        AC_LANG_SOURCE(
+            [[
+                int main() {
+                #ifndef _FORTIFY_SOURCE
+                    return 0;
+                #else
+                    this_is_an_error;
+                #endif
+                }
+            ]]
+        )], [
+            AC_MSG_RESULT([yes])
+            CFLAGS="$CFLAGS -D_FORTIFY_SOURCE=2"
+        ], [
+            AC_MSG_RESULT([no])
+    ])
+])