Win32: added a variable to specify compiler.

This allows to set a different one from command line when needed.
For example, to configure nginx with gcc as a compiler one could
use "make -f misc/GNUmakefile win32 CC=gcc".

misc/GNUmakefile

diff --git a/misc/GNUmakefile b/misc/GNUmakefile
index 9ab4193..064fac1 100644
--- a/misc/GNUmakefile
+++ b/misc/GNUmakefile
@@ -4,6 +4,7 @@
 NGINX =		nginx-$(VER)
 TEMP =		tmp
 
+CC =		cl
 OBJS =		objs.msvc8
 OPENSSL =	openssl-1.0.2j
 ZLIB =		zlib-1.2.8
@@ -47,7 +48,7 @@
 
 win32:
 	./auto/configure						\
-		--with-cc=cl						\
+		--with-cc=$(CC)						\
 		--builddir=$(OBJS)					\
 		--with-debug						\
 		--prefix= 						\