Add libgif port

[saschanaz]

This can successfully build JPEG XL reference software with GIF support.

[kripken]

What is the relationship between JPEG XL and libgif? (Sorry, I don't know much about this area - when I did a search for libgif I found "giflib" which doesn't mention JPEG XL at all, is that something else?)

[saschanaz]

giflib is libgif as zlib is libz. Maybe it should be giflib here to follow zlib.

[saschanaz]

What is the relationship between JPEG XL and libgif

https://gitlab.com/wg1/jpeg-xl depends on giflib to decode gif. I just intended to say I tested this with a real world project.

[kripken]

I see, thanks @saschanaz !

Ok, looks good, just please add a test before we land. Can be similar to the test for libjpeg for example,

emscripten/tests/test_other.py

Lines 1501 to 1506 in 63422ac

	def test_libjpeg(self):
	shutil.copyfile(path_from_root('tests', 'screenshot.jpg'), 'screenshot.jpg')
	building.emcc(path_from_root('tests', 'jpeg_test.c'), ['--embed-file', 'screenshot.jpg', '-s', 'USE_LIBJPEG'], output_filename='a.out.js')
	self.assertContained('Image is 600 by 450 with 3 components', self.run_js('a.out.js', args=['screenshot.jpg']))