gmp: fix build on libc++ / clang on 10.9

Fixes #21259.
brew-test-bot: use REXML to write JUnit file.
detect_cxx_stdlibs: Only detect for dylibs

Fixes #23115.
cfitsio: add mirror

Closes #23135.
guile: promote 2.0.9 to stable

We held back on 2.0 as stable way back in #5127 because lilypond didn't
work with it, but now everything in core works with guile 2.0.9.
texxmacs 1.0.7.20