Port to GCC 4.9.0 with link time optimization.

* configure.ac (CFLAGS): With link time optimization,
use -ffat-lto-objects if supported; otherwise Emacs won't
build with GCC 4.9.0.

Fixes: debbugs:17806

ChangeLog

+2014-06-20  Paul Eggert  <eggert@cs.ucla.edu>
+
+	Port to GCC 4.9.0 with link time optimization (Bug#17806).
+	* configure.ac (CFLAGS): With link time optimization,
+	use -ffat-lto-objects if supported; otherwise Emacs won't
+	build with GCC 4.9.0.
+
 2014-06-14  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Port part of the AIX fix to Solaris (Bug#17598).

configure.ac

@@ -777,6 +777,12 @@ if test "${enableval}" != "no"; then
    AC_MSG_RESULT([$ac_lto_supported])
    if test "$ac_lto_supported" = "yes"; then
       CFLAGS="$CFLAGS $LTO"
+      dnl The following is needed for GCC 4.9.0.  The GCC 4.9.0 release notes
+      dnl suggest that instead of -ffat-lto-objects we should use gcc-ar and
+      dnl gcc-ranlib in place of ar and ranlib, but gcc-ar makes /usr/bin/ar
+      dnl dump core on Fedora 20, so play it safe for now.
+      gl_COMPILER_OPTION_IF([-ffat-lto-objects],
+	[CFLAGS="$CFLAGS -ffat-lto-objects"])
    fi
 fi)