Support pkg-config installation

[no92]

This adds a backwards-compatible way to allow for installing uACPI sources onto a system or sysroot while providing a uacpi.pc file for discoverability.

Integrating this into a project using meson could look like this:

uacpi = dependency('uacpi', method: 'pkg-config')
uacpi_includes = include_directories(uacpi.get_variable('includedir'))

uacpi_sources = files()
foreach f : uacpi.get_variable('sourcefiles').split(';')
	uacpi_sources += files(uacpi.get_variable('installdir') / f)
endforeach