compiler-gcc.h: Add gcc-recommended GCC_VERSION macro

Throughout compiler*.h, many version checks are made.  These can be
simplified by using the macro that gcc's documentation recommends.
However, my primary reason for adding this is that I need bug-check
macros that are enabled at certain gcc versions and it's cleaner to use
this macro than the tradition method:

  #if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ => 2)

If you add patch level, it gets this ugly:

  #if __GNUC__ > 4 || (__GNUC__ == 4 && (__GNUC_MINOR__ > 2 || \
      __GNUC_MINOR__ == 2 __GNUC_PATCHLEVEL__ >= 1))

As opposed to:

  #if GCC_VERSION >= 40201

While having separate headers for gcc 3 & 4 eliminates some of this
verbosity, they can still be cleaned up by this.

See also:

  http://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.htmlSigned-off-by: Daniel Santos <daniel.santos@pobox.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Joe Perches <joe@perches.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compiler-gcc.h

@@ -5,6 +5,9 @@
 /*
  * Common definitions for all gcc versions go here.
  */
+#define GCC_VERSION (__GNUC__ * 10000 \
+		   + __GNUC_MINOR__ * 100 \
+		   + __GNUC_PATCHLEVEL__)
 
 
 /* Optimization barrier */