Allow setting GIT_VERSION

Makefile.libretro

@@ -29,7 +29,7 @@ else ifneq ($(findstring MINGW,$(shell uname -a)),)
 endif
 
 TARGET_NAME := 2048
-GIT_VERSION := " $(shell git rev-parse --short HEAD || echo unknown)"
+GIT_VERSION ?= " $(shell git rev-parse --short HEAD || echo unknown)"
 ifneq ($(GIT_VERSION)," unknown")
 	CFLAGS += -DGIT_VERSION=\"$(GIT_VERSION)\"
 endif