block-sha1: guard gcc extensions with __GNUC__

With this, the code should now be portable to any C compiler.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

block-sha1/sha1.c

@@ -9,7 +9,7 @@
 
 #include "sha1.h"
 
-#if defined(__i386__) || defined(__x86_64__)
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
 
 /*
  * Force usage of rol or ror by selecting the one with the smaller constant.
@@ -54,7 +54,7 @@
 
 #if defined(__i386__) || defined(__x86_64__)
   #define setW(x, val) (*(volatile unsigned int *)&W(x) = (val))
-#elif defined(__arm__)
+#elif defined(__GNUC__) && defined(__arm__)
   #define setW(x, val) do { W(x) = (val); __asm__("":::"memory"); } while (0)
 #else
   #define setW(x, val) (W(x) = (val))