wincred: avoid overwriting configured variables

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Acked-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/credential/wincred/Makefile

@@ -1,12 +1,12 @@
 all: git-credential-wincred.exe
 
-CC = gcc
-RM = rm -f
-CFLAGS = -O2 -Wall
-
 -include ../../../config.mak.autogen
 -include ../../../config.mak
 
+CC ?= gcc
+RM ?= rm -f
+CFLAGS ?= -O2 -Wall
+
 prefix ?= /usr/local
 libexecdir ?= $(prefix)/libexec/git-core