Fix build for older gcc

With gcc 4.3.4 I'm seeing the following warning failure

cc1: warnings being treated as errors
cc1: error: -funit-at-a-time is required for inlining of functions
that are only called once [-Wdisabled-optimization]

Add -funit-at-a-time to WARN_CFLAGS.

m4/virt-compile-warnings.m4

@@ -112,6 +112,7 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
         gl_WARN_ADD([-fexceptions])
         gl_WARN_ADD([-fasynchronous-unwind-tables])
         gl_WARN_ADD([-fdiagnostics-show-option])
+        gl_WARN_ADD([-funit-at-a-time])
 
         if test "$enable_compile_warnings" = "error"
         then