make: allow to change a compiler

clang-analyzer sets the environment variable CC
scan-build -plist-html --use-analyzer /usr/bin/clang make
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

Makefile

@@ -32,7 +32,7 @@ CSCOPE		:= cscope
 TAGS		:= ctags
 RM		:= rm -f
 LD		:= ld
-CC		:= gcc
+CC		?= gcc
 ECHO		:= echo
 NM		:= nm
 AWK		:= awk