graphics/libheif: Clean up Makefile, add JPEG by default

[cem]

Summary

Mostly non-functional change adding options to various features. In addition
to adding a JPEG option, add it to OPTIONS_DEFAULT. This is useful because
heif-convert is one of the few ways to convert a .heic photo to .jpeg, and both
are commonly used for similar media where .png does not necessarily make sense.

Correct the LICENSE for the examples code -- in 1.3.2, it is GPLv3+. (Though
in later git versions, they are relicensed to MIT.)

Test Plan

I did some local builds with options disabled and DEVELOPER yes and check-plist
and pkg contents seemed ok.

heif-convert from a .heic I had on hand to jpg succeeded.

I don't see the maintainer on Phabricator; I will attempt to contact him by
email.