audio/mumble: switch from USE_GITHUB to pre-rolled release and fix build on ARCHes other than x86

1. Switch from USE_GITHUB to pre-rolled release.
2. Use CMake to install the files instead of using custom do-install target.
3. Unbundle devel/microsoft-gsl.
4. Fix build and keep possibility to build 32bit overlay on other ARCHes than x86.
5. "Pet portclippy".

Ideas, patches and testing: fuz, jhale, pkubaj.

PR: 273203 272881
Approved by: tcberner
Differential Revision: https://reviews.freebsd.org/D41604
MFH: 2023Q3