Fix the definition of 'assume' for GCC older than 4.5.

 src/conf_post.h (__has_builtin): Define to zero, if undefined, on
 all platforms, not just for clang.

src/ChangeLog

+2013-09-23  Eli Zaretskii  <eliz@gnu.org>
+
+	* conf_post.h (__has_builtin): Define to zero, if undefined, on
+	all platforms, not just for clang.
+
 2013-09-23  Jan Djärv  <jan.h.d@swipnet.se>
 
 	* filelock.c (lock_file_1): Rearrange to remove compiler warning

src/conf_post.h

@@ -248,10 +248,8 @@ extern void _DebPrint (const char *fmt, ...);
 # define FLEXIBLE_ARRAY_MEMBER 1
 #endif
 
-#ifdef __clang__
-# ifndef __has_builtin
-#  define __has_builtin(x) 0
-# endif
+#ifndef __has_builtin
+# define __has_builtin(x) 0
 #endif
 
 /* assume(cond) tells the compiler (and lint) that a certain condition