Correct BUILDLINK_PKGSRCDIR.mupdf path and bump BUILDLINK_API_DEPENDS.mupdf
to 1.3.
The package was (at least) also tested by Makoto Fujiwara on a different port
(amd64), different version (6.99.23) and different compiler (clang).
Nobody complained about the quality of workarounds so delete the "TODO"
section... Now this file could be called "DONE" and it is just a reference for
future updates of mupdf.

XXX: Please attach this file in the commit message when importing wip/mupdf
in pkgsrc.