Do not enable _FORTIFY_SOURCE without optimization

With glibc-2.16, using -D_FORTIFY_SOURCE requires that optimization (-O)
be used or it will prodice a warning message. Enable -Werror in our
test for _FORTIFY_SOURCE support to catch when a users specifies CFLAGS
without optimization.

The line to set CFLAGS="" when no CFLAGS are specified (either due to
being unset or geniunely empty) is required as autoconf will use
"-O2 -g" for its tests by defult when CFLAGS is unset, but will not add
them to the CFLAGS used...

Signed-off-by: Allan McRae <allan@archlinux.org>

configure.ac

@@ -66,6 +66,11 @@ LT_INIT
 LIB_VERSION=`expr lib_current - lib_age`.lib_age.lib_revision
 LIB_VERSION_INFO="lib_current:lib_revision:lib_age"
 
+# Respect empty CFLAGS during compiler tests
+if test "x$CFLAGS" != "x"; then
+  CFLAGS=""
+fi
+
 # Set subsitution values for version stuff in Makefiles and anywhere else,
 # and put LIB_VERSION in config.h
 AC_SUBST(LIB_VERSION)

m4/acinclude.m4

@@ -40,11 +40,15 @@ AC_DEFUN([GCC_STACK_PROTECT_CC],[
 
 dnl GCC_FORTIFY_SOURCE_CC
 dnl checks -D_FORTIFY_SOURCE with the C compiler, if it exists then updates
-dnl CFLAGS
+dnl CPPFLAGS
 AC_DEFUN([GCC_FORTIFY_SOURCE_CC],[
   AC_LANG_ASSERT(C)
   if test "X$CC" != "X"; then
     AC_MSG_CHECKING(for FORTIFY_SOURCE support)
+    fs_old_cppflags="$CPPFLAGS"
+    fs_old_cflags="$CFLAGS"
+    CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"
+    CFLAGS="$CFLAGS -Werror"
     AC_TRY_COMPILE([#include <features.h>], [
       int main() {
       #if !(__GNUC_PREREQ (4, 1) )
@@ -54,10 +58,12 @@ AC_DEFUN([GCC_FORTIFY_SOURCE_CC],[
       }
     ], [
       AC_MSG_RESULT(yes)
-      CPPFLAGS="$CPPFLAGS -D_FORTIFY_SOURCE=2"
+      CFLAGS="$df_old_cflags"
     ], [
       AC_MSG_RESULT(no)
-  ])
+      CPPFLAGS="$df_old_cppflags"
+      CFLAGS="$df_old_cflags"
+    ])
   fi
 ])