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.

[Ryzee119]

Thanks, I tested this on a couple of my cmake projects. After adding the new dependency on pkgconf, all worked as expected