Generate pkg-config files directly

[Kangie]

I recently ported the openrc-plymouth-plugin to use the Meson build system. As part of that I made a seemingly sane decision to install to get_option('libdir'), 'rc' / 'plugins' based on PFL listings among other things.

Unfortunately this assumption broke on certain Gentoo configurations.

This PR attempts to address this by providing a rc_path pkgconfig variable that can be queried so that plugins (etc) don't have to make assumptions.

To do this we update to using the meson pkgconfig module to generate pkgconfig files from whole cloth and drop the old configure_file path.

• Remove 'pkgconfig' subdir
• use meson 'pkgconfig' module to generate and install appropriate .pc files when required.
• Raise minimum meson version to 0.56.0 (pkgconfg variables as dict)
• add rc_path variable

Plymouth Plugin Bug: https://bugs.gentoo.org/935378

[Kangie]

/tmp/openrc/usr/lib64/pkgconfig/einfo.pc
prefix=/usr
includedir=${prefix}/include
libdir=${prefix}/lib64

Name: einfo
Description: Pretty console informational display
Version: 0.54
Libs: -L/lib64 -leinfo
Cflags: -I${includedir}/einfo

/tmp/openrc/usr/lib64/pkgconfig/OpenRC.pc
prefix=/usr
includedir=${prefix}/include
libdir=${prefix}/lib64

rc_path=/lib/rc

Name: OpenRC
Description: Universal init system
Version: 0.54
Libs: -L/lib64 -lrc
Cflags: -I${includedir}/openrc