Add dos compilation

Makefile.libretro

@@ -201,6 +201,14 @@ else ifeq ($(platform), vita)
 	AR = arm-vita-eabi-ar$(EXE_EXT)
    PLATFORM_DEFINES := -DVITA
    STATIC_LINKING = 1
+# DOS
+else ifeq ($(platform), dos)
+	TARGET := $(TARGET_NAME)_libretro_$(platform).a
+	CC = i586-pc-msdosdjgpp-gcc
+	AR = i586-pc-msdosdjgpp-ar
+	CXX = i586-pc-msdosdjgpp-g++
+	PLATFORM_DEFINES += -march=i386
+	STATIC_LINKING=1
 
 # CTR (3DS)
 else ifeq ($(platform), ctr)