[PATCH] Add -D_FORTIFY_SOURCE=2 to CFLAGS if possible.

[Mark Wielaard <mark@klomp.org>]

Some distros now add -D_FORTIFY_SOURCE=2 by default and we have missed
some issues in the past caught by it. Add it to CFLAGS if possible.
The configure check will make sure that it doesn't conflict with any
other CFLAGS already defined.

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 ChangeLog    |  4 ++++
 configure.ac | 21 +++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index a587b43..b50f79e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2017-02-09  Mark Wielaard  <mark@klomp.org>
+
+	* configure.ac: Add check for adding -D_FORTIFY_SOURCE=2 to CFLAGS.
+
 2017-01-12  Mark Wielaard  <mark@klomp.org>
 
 	* configure.ac: Define PACKAGE_URL for older autoconf.
diff --git a/configure.ac b/configure.ac
index 07ad592..46055f2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -159,6 +159,27 @@ AC_CACHE_CHECK([whether fts.h is bad when included (with LFS)], ac_cv_bad_fts,
 		     ac_cv_bad_fts=no, ac_cv_bad_fts=yes)])
 AS_IF([test "x$ac_cv_bad_fts" = "xyes"], [CFLAGS="$CFLAGS -DBAD_FTS=1"])
 
+# See if we can add -D_FORTIFY_SOURCE=2. Don't do it if it is already
+# (differently) defined or if it generates warnings/errors because we
+# don't use the right optimisation level (string.h will warn about that).
+AC_MSG_CHECKING([whether to add -D_FORTIFY_SOURCE=2 to CFLAGS])
+case "$CFLAGS" in
+  *-D_FORTIFY_SOURCE=2*)
+    AC_MSG_RESULT([no, already there])
+    ;;
+  *)
+    save_CFLAGS="$CFLAGS"
+    CFLAGS="-D_FORTIFY_SOURCE=2 -Werror $CFLAGS"
+    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
+      #include <string.h>
+      int main() { return 0; }
+    ]])], [ AC_MSG_RESULT([yes])
+            CFLAGS="-D_FORTIFY_SOURCE=2 $save_CFLAGS" ],
+          [ AC_MSG_RESULT([no])
+            CFLAGS="$save_CFLAGS"])
+  ;;
+esac
+
 dnl enable debugging of branch prediction.
 AC_ARG_ENABLE([debugpred],
 AS_HELP_STRING([--enable-debugpred],[build binaries with support to debug branch prediction]),
-- 
1.8.3.1