mingw: stop hard-coding CC = gcc

This is no longer true in general, not with supporting Clang out of the
box.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

config.mak.uname

@@ -761,7 +761,6 @@ ifeq ($(uname_S),MINGW)
 		ETC_GITCONFIG = ../etc/gitconfig
 		ETC_GITATTRIBUTES = ../etc/gitattributes
         endif
-	CC = gcc
 	COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -fstack-protector-strong
 	EXTLIBS += -lntdll
 	EXTRA_PROGRAMS += headless-git$X