Allow to use the external mupdf package

[yurivict]

Is your feature request related to a problem? Please describe.
Patching can be done in one place only

Describe the solution you'd like
Have a build-time option to use externally installed mupdf.

Describe alternatives you've considered
n/a

Additional context
I had to patch the FreeBSD port just to enable debug info in the bundled binary mupdf code.

[julian-smith-artifex-com]

Have you seen https://pymupdf.readthedocs.io/en/latest/packaging.html? This explains how to build with an externally installed mupdf.

(Despite the title Packaging for Linux distributions, it should apply equally to FreeBSD. I'd be happy to update the page with any freebsd-specific information that is required.)

Also see description of PYMUPDF_SETUP_MUPDF_BUILD_TYPE in setup.py - this can be used to do a debug build of mupdf.