[WIP] Link to system ls-qpack, fall back to vcpkg

[CL-Jeremy]

This pull request adds support for vcpkg for building wheels for distribution over PyPI, while opportunistically linking to OS-provided packages, all by using pkgconf via matze/pkgconfig.

The current solution, notably the use of glob.glob() for prefix detection, is arguably fragile (would break if someone copied over the vcpkg_installed folder with installed packages from another platform). Probably a clean-up step before the build this would suffice, though.

Also: since the maintainer from Fedora renamed the pkg-config package configuration file (Oct 31 2023, https://src.fedoraproject.org/rpms/ls-qpack/c/bc56a0c45fff32c32d157c901e6a57cf1aadc9a6.patch) while waiting their patch (Oct 22 2023, litespeedtech/ls-qpack#55) to be merged upstream, this currently won't work on Fedora. Compare the file names:

	library name	.pc file name
Fedora	liblsqpack.so	ls-qpack.pc
Upstream	libls-qpack.a	lsqpack.pc

I have no experience in such communication regarding breaking name changes, so I cannot make a decision yet.

Closes: #42

PS: This is still a work in progress and should wait until the ad-hoc patches have been upstreamed, or when better solutions are found and implemented here.

[CL-Jeremy]

Interestingly, this seems to have worked on macOS and Linux, but on Windows, the (supposedly successfully compiled and linked) DLL file could not be linked, and upon inspection with dumpbin /linkermember revealed no exported symbols...

[CL-Jeremy]

Just tested on MSYS2 under Windows. Neither UCRT64 nor MinGW64 works, but solving that would need much more effort on the vcpkg side (as the support is non-official on these platforms). It would be much easier just to add ls-qpack into MSYS2 considering the fact that the user would be in GCC/Cygwin realm anyway.

Edit: Sorry, confused the Cygwin-based packages (no prefix) with MinGW ones (prefix: mingw-w64-). Probably the user would just use the packaged one anyway, but it would be really nice if we could actually decouple this from the main library.