This is motivated by https://github.com/msys2/MINGW-packages/pull/20591
where flang v18 stopped supporting the -pipe flag which is passed to
it via LDFLAGS. v17 still supported it and it's not clear where and when
it was changed exactly.
Let's remove it from LDFLAGS because:
* it's not clear if -pipe actually does anything for linking
(it's still passed via CFLAGS/CXXFLAGS)
* other distros like Arch/Gentoo also only add it to CFLAGS/CXXFLAGS
* of the above flang issue
* Add a dependency on zstd so that makepkg can use it
* Remove the upx option from the config because it was removed
in pacman: https://git.archlinux.org/pacman.git/commit/?id=1a29744d0d
* Various other syncs and adjustments for zstd
.. in the face of using `git am` to apply patches.
We set `GIT_COMMITTER_NAME` and `GIT_COMMITTER_EMAIL` to fixed
values and provide a `gitam_mkpkg` function that passes
`--committer-date-is-author-date`
The means we can generate packages with the same, correct sha1
without needing to mess about with `git reset` (and thus getting
the old sha1 before our patches were applied).
Also, removed the `-j1` from make since it doesn't seem to be
needed anymore. Perhaps upstream fixed a race condition?
