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

@@ -763,7 +763,6 @@ ifeq ($(uname_S),MINGW)
 		COMPAT_CFLAGS += -D_USE_32BIT_TIME_T
 		BASIC_LDFLAGS += -Wl,--large-address-aware
         endif
-	CC = gcc
 	COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -DDETECT_MSYS_TTY \
 		-fstack-protector-strong
 	EXTLIBS += -lntdll