We want to include via <apk/$headerfile> and not via <$headerfile>,
so we want to add an include statement for the dir the apk folder
which contains the headers is contained in.

- always create .so and .a with the apk libary code
- create .pc file
- install headers and above mentioned files

Maintainers edit: merged commits, cleaned up sed script logic,
and few other minor fixes.

That list is needed in multiple places, so it simplifies
things a bit.

This commit sets up the build system to compile man pages with scdoc,
and adds the first man page: apk(8).