Split MANGLE macro into LOCAL_MANGLE AND EXTERN_PREFIX parts

so that LOCAL_MANGLE can be used without EXTERN_PREFIX.
patch by Alexander Strange, astrange ithinksw com

Originally committed as revision 12558 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/internal.h

@@ -120,11 +120,13 @@
 
 // Use rip-relative addressing if compiling PIC code on x86-64.
 #if defined(ARCH_X86_64) && defined(PIC)
-#    define MANGLE(a) EXTERN_PREFIX #a"(%%rip)"
+#    define LOCAL_MANGLE(a) #a "(%%rip)"
 #else
-#    define MANGLE(a) EXTERN_PREFIX #a
+#    define LOCAL_MANGLE(a) #a
 #endif
 
+#define MANGLE(a) EXTERN_PREFIX LOCAL_MANGLE(a)
+
 /* debug stuff */
 
 /* dprintf macros */