Multi-output `includedir` magic for `pkg-config` was not working, as not
only `libdir` was affected by `postPatch` script. And absolute path to
include directory is not compatibe with this magic either. So the
pkg-config file contained a path to non-existing directory (`include`,
but in the main `out` output).
The only package to use this library is openscad, but it's currently
not picking it - will fix it in a separate PR.
