Add support for mapnik falling back to pkg-config if mapnik-config is not present

[nmaludy]

Whenever the Mapnik makes a new release, the build system supports CMake which doesn't generate the mapnik-config tool: mapnik/mapnik#4330

It would be great if mod_tile could fallback to using pkg-config to find the library if mapnik-config is not present.

[hummeltech]

Hey @nmaludy, great thinking!

I was working on resolving #304 & this issue in this branch, you can see it passing in GitHub Actions as well.

[nmaludy]

Awesome, great work!