libports/sdl_image: enable XPM, add include/SDL

Some SDL applications expect the SDL_image headers in include/SDL to be
reachable without the SDL/ prefix. This patch adds the corresponding
search path. Furthermore it enables support for XPM images.

repos/libports/lib/import/import-sdl_image.mk

@@ -1 +1,2 @@
-INC_DIR += $(call select_from_ports,sdl_image)/include
+INC_DIR += $(call select_from_ports,sdl_image)/include \
+           $(call select_from_ports,sdl_image)/include/SDL

repos/libports/lib/mk/sdl_image.mk

@@ -4,7 +4,7 @@ SRC_C = $(notdir $(wildcard $(SDL_IMAGE_PORT_DIR)/src/lib/sdl_image/IMG*.c))
 
 LIBS += libc libm sdl jpeg libpng zlib
 
-SUPPORTED_FORMATS = PNG JPG TGA PNM
+SUPPORTED_FORMATS = PNG JPG TGA PNM XPM
 
 CC_OPT += $(addprefix -DLOAD_,$(SUPPORTED_FORMATS))