[noupstream] disable M4 in GMP

... as we have been doing in tools/Makefile for ages.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6594044
diff --git a/gmp-4.3.1/configure b/gmp-4.3.1/configure
index e8d99a7..e3f2eb0 100755
--- a/gmp-4.3.1/configure
+++ b/gmp-4.3.1/configure
@@ -29082,8 +29082,6 @@
 echo "define(<M4WRAP_SPURIOUS>,<$gmp_cv_m4_m4wrap_spurious>)" >> $gmp_tmpconfigm4
 
 
-else
-  M4=m4-not-needed
 fi
 
 # Only do the GMP_ASM checks if there's a .S or .asm wanting them.