[libpq,libxslt] Update libpq, fix misc issues

[dg0yt]

libxslt

Fixes libname in the installed pc files for msvc triplets.

libpq

Update to 15.3.
Fixes feature zstd pulling in lz4 libs.
da1fc66 fixes #31242 (to be tested).
29a66ae fixes #30327 (to be tested).
a4e83a5 fixes #30099.
Fixes building the programs (libpq[client]) with all possible features enabled.
Fixes building libpq[python] on static non-msvc triplets using pkg-config.
Establishes consistency between msvc and non-msvc triplets by always installing libecpg.
Fixes race conditions in non-msvc builds.
Mitigates winflex races in msvc builds.
Cleanup.

Notes

References for Upstream platform support:
https://www.postgresql.org/about/featurematrix/#platforms
https://buildfarm.postgresql.org/cgi-bin/show_status.pl

• uwp is unsupported upstream. Unchanged.

• arm64-windows is unsupported upstream. Patches were extended. I don't know if the binaries were healthy before this PR, withouth the spin_delay patch, and I'm unable to test if they are healthy now.
  Alternative (seriously): Drop patched-in arm64-windows support. Affects downstream ports (e.g. gdal).

• *-windows-static-* is unsupported upstream. Patches were extended. I don't know if the binaries were healthy before this PR, e.g. without dllexport/dllimport changes. Maybe this was subject to feature configuration. At least the client programs build now, with all possible features enabled.

• The msvc builds don't provide exported configurations (pkgconfig), and the cmake wrapper doesn't yet cover all features. This can cause downstream ports (e.g gdal) to fail because they are unaware of the full set of transitive link libraries pulled in via optional features.
  Not a regression and out of scope for this PR. (NB: Upstream doesn't seem to support static builds.)

• On windows, building the programs (libpq[client]) suffers from [mesa] win_FLEX.exe has races in %TEMP% that cause sporadic build failures. #29139. Not a regression.
  This is now mitigated by not using the -m msbuild option when building with the opt-in client feature. Build-time effect in vcpkg CI:

  • default features: < 1 min
  • all features with -m: 2 min if successful
  • all features w/o -m: 5 min

  The -m effect may be less drastic on user machines with less CPU cores.
  In vcpkg CI, the client feature won't be build when this PR is complete, so it doesn't hurt there.

• There is also a Github mirror: https://github.com/postgres/postgres
  It would allow for --head builds.

• The coming version 16 seems to support meson.

Checklist

• Changes comply with the maintainer guide
• SHA512s are updated for each updated download
• The "supports" clause reflects platforms that may be fixed by this new version
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.

[Cheney-W]

Features passed with following triplets:
x86-windows
x64-windows
x64-windows-static