Allow LIBRETRO_COMM_DIR override

The location of libretro-common may be overriden by setting the
LIBRETRO_COMM_DIR variable via the command line.

Makefile.common

+ifeq ($(LIBRETRO_COMM_DIR),)
 LIBRETRO_COMM_DIR  = $(CORE_DIR)/libretro-common
+endif
 LIBFLAC_DIR    = $(CORE_DIR)/deps/libFLAC
 LIBVORBIS_DIR  = $(CORE_DIR)/deps/libvorbis
 LIBOGG_DIR     = $(CORE_DIR)/deps/libogg