Allow setting GIT_VERSION

Author: orbea

Makefile

@@ -92,7 +92,7 @@ LDFLAGSEMULATOR =
 
 CCOMFLAGS  += -D__LIBRETRO__
 
-GIT_VERSION := " $(shell git rev-parse --short HEAD || echo unknown)"
+GIT_VERSION ?= " $(shell git rev-parse --short HEAD || echo unknown)"
 ifneq ($(GIT_VERSION)," unknown")
 	CCOMFLAGS += -DGIT_VERSION=\"$(GIT_VERSION)\"
 endif