Pkgconfig CMake compatibility & switch to pkgconf

[thrimbor]

The last two commits are from #631, with only the commit title updated. They make it possible for CMake to find the libraries to link against via pkg-config.

These two commits had the problem that they fix CMake, but by doing so are breaking direct usage of nxdk-pkg-config (and probably meson) because the emitted linker arguments are in ld syntax, while we rely on the syntax of link.exe.

This PR circumvents this issue by switching from whatever pkg-config the host OS provides to explicitly requiring pkgconf (which is the default pkg-config implementation on Arch, Alpine and probably others anyway).
pkgconf provides a --msvc-syntax parameter that can be used to get the required link.exe-style linker parameters. This would add an additional .lib suffix to the library paths (resulting in things like libjpeg.lib.lib), so I added a small sed command to fix that.

[JayFoxRox]

I'm not sure I follow the PR description.

I thought the .lib.lib only happens when using --msvc-syntax, but now --msvc-syntax is absent?
(Also, why does it produce .lib.lib? This feels like a bug - was it reported?)

[thrimbor]

I thought the .lib.lib only happens when using --msvc-syntax, but now --msvc-syntax is absent?

It does, but passing in --msvc-syntax is up to the user. Generally, a user will want to specify it for the linker (which follows link.exe parameter syntax), but not for the compiler (which follows gcc/clang syntax), and not for CMake (which expects ld syntax).

(Also, why does it produce .lib.lib? This feels like a bug - was it reported?)

Not a bug, it matches freedesktop's pkg-config implementation.
To convert the ld to link.exe syntax, the -l is stripped and a .lib added (for example, on my host system pkg-config libpng --libs produces -lpng16, whereas with --msvc-syntax I get png16.lib instead, because with MSVC libraries by convention have a .lib ending).
We already have our parameters end in .lib, with the one added by pkgconf we get .lib.lib