Always compile with gcc

This avoids the problem where the cc.exe of *MSys2* would be picked to
compile Git Cheetah in Git for Windows 2.x' SDK because there is no
MinGW cc.exe.

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

common/Makefile

@@ -1,6 +1,8 @@
 ifneq (,$(W64))
 cross = /src/mingw-w64/sysroot/bin/x86_64-w64-mingw32-
 CC = $(cross)gcc.exe
+else
+CC = gcc
 endif
 
 MODULES=debug.c exec.c menuengine.c cheetahmenu.c date.c \