meson: Use configure_file for pc files.

Meson's pkg.generate() is not suitable for header only libraries and using configure_file() allows for more fine tuned control. This also makes the meson and autotools builds more in sync where they both use the same .pc.in files.

Some inherent issues:

• meson doesn't seem to support exec_prefix so this will just use prefix instead.
• meson doesn't allow installing the includedir outside of the prefix.

Note my main goal is to make sure commit 91c1c8e1 also applies to the meson build. When asking in #mesonbuild @ freenode I was told to try using configure_file() instead.