build: Produce correct pc file with absolute libdir

build: Produce correct pc file with absolute libdir

The CMAKE_INSTALL_INCLUDEDIR and CMAKE_INSTALL_LIBDIR variables
can be absolute paths so simply appending them to \${prefix}
might not work.

Unfortunately, CMake does not have simple function for joining paths [1]
so I had to write my own suboptimal one.

It somewhat handles the following cases

- prefix absolute + libdir relative
- prefix absolute + libdir absolute but not under prefix

but for

- prefix absolute + libdir absolute & under prefix

we would want to strip the prefix and make the libdir relative
so we can make it relative to the pkg-config variables.

Basically, we want the equivalent of the following Python function:

	import os.path
	from pathlib import PurePath

	if os.path.isabs(libdir) and os.path.commonpath([libdir, prefix]) == str(PurePath(prefix)):
	    # Convert path to be relative to prefix
	    skip = len(prefix) + 1
	    libdir = libdir[skip:]

	return os.path.join('${prefix}', libdir)

This would be non-trivial to replicate in CMake scripts as far as I can tell.

[1]: https://gitlab.kitware.com/cmake/cmake/issues/19568