libretro: allow mingw cross compile

EMULib/Rules.gcc

@@ -7,14 +7,14 @@ LIB68K	= $(BASEDIR)/M68000
 LIBGBZ	= $(BASEDIR)/GBZ80
 PRIVATE	= $(BASEDIR)/Private
 
-CXX	= g++
-CC	= gcc
-CPP	= cpp
-MOC	= moc
-SDT2GAS	= perl $(EMULIB)/Tools/sdt2gas.pl
-CFLAGS	= -I. -I.. -I$(EMULIB)
-DEFINES = -DZLIB
-LIBS    = -lz 
+CXX	?= g++
+CC	?= gcc
+CPP	?= cpp
+MOC	?= moc
+SDT2GAS	?= perl $(EMULIB)/Tools/sdt2gas.pl
+CFLAGS	+= -I. -I.. -I$(EMULIB)
+DEFINES += -DZLIB
+LIBS    += -lz 
 
 # EMULib sound, console, and other utility functions 
 OBJECTS	= $(EMULIB)/EMULib.o $(EMULIB)/Sound.o   \

Makefile

@@ -472,8 +472,8 @@ else ifneq (,$(findstring windows_msvc2017,$(platform)))
 else
 
    TARGET := $(TARGET_NAME)_libretro.dll
-   CC = gcc
-   CC_AS = gcc
+   CC ?= gcc
+   CC_AS ?= gcc
    SHARED := -shared -static-libgcc -static-libstdc++ -Wl,-no-undefined -Wl,-version-script=link.T
 endif