Fix libretro-imageviewer standalone compilation

cores/libretro-imageviewer/Makefile

@@ -1,21 +1,43 @@
-image_core.so: image_core.c
-	gcc \
-		-g \
-		-DHAVE_STB_IMAGE \
-		image_core.c \
-		-I../../libretro-common/include/ \
-		-I../../deps/stb/ \
-		../../libretro-common/compat/compat_strcasestr.c \
-		../../libretro-common/compat/compat_strl.c \
-		../../libretro-common/file/file_path.c \
-		../../libretro-common/file/file_path_io.c \
-		../../libretro-common/file/retro_dirent.c \
-		../../libretro-common/lists/dir_list.c \
-		../../libretro-common/lists/string_list.c \
-		../../libretro-common/streams/file_stream.c \
-		../../libretro-common/vfs/vfs_implementation.c \
-		-shared \
-		-fPIC \
-		-Wl,--no-undefined \
-		-lm \
-		-o image_core.so
+CC      ?= gcc
+CFLAGS  ?= -g
+LDFLAGS ?=
+
+ifneq ($(SANITIZER),)
+   CFLAGS  := -fsanitize=$(SANITIZER) -fno-omit-frame-pointer $(CFLAGS)
+   LDFLAGS := -fsanitize=$(SANITIZER) $(LDFLAGS)
+endif
+
+SOURCES := \
+	image_core.c \
+	../../libretro-common/compat/compat_strcasestr.c \
+	../../libretro-common/compat/compat_strl.c \
+	../../libretro-common/compat/fopen_utf8.c \
+	../../libretro-common/encodings/encoding_utf.c \
+	../../libretro-common/file/file_path.c \
+	../../libretro-common/file/file_path_io.c \
+	../../libretro-common/file/retro_dirent.c \
+	../../libretro-common/lists/dir_list.c \
+	../../libretro-common/lists/string_list.c \
+	../../libretro-common/streams/file_stream.c \
+	../../libretro-common/time/rtime.c \
+	../../libretro-common/vfs/vfs_implementation.c
+
+INCLUDES := \
+	-I../../libretro-common/include/ \
+	-I../../deps/stb/
+
+DEFINES := -DHAVE_STB_IMAGE
+
+TARGET := image_core.so
+
+all: $(TARGET)
+
+$(TARGET): $(SOURCES)
+	$(CC) $(CFLAGS) $(DEFINES) $(INCLUDES) $(SOURCES) \
+		-shared -fPIC -Wl,--no-undefined -lm \
+		$(LDFLAGS) -o $(TARGET)
+
+clean:
+	rm -f $(TARGET)
+
+.PHONY: all clean