find_library() with dirs kwarg falls back to system lib dir

In the documentation of `compiler.find_library()`, it states that the `dirs` kwarg should override the system lib dir. But given a `dirs` argument that refers to a non-existent directory, the function goes to the system lib dir and finds a library with the corresponding name.

For our build we need it to fail instead. If we think the library was found according to the `dirs` argument, then we will `include_directories` using a special relative path inside that `dirs` area. This of course does not work where the library was (unexpectedly) found in the system lib dir.

Start at the implementation and documentation for compiler.find_library(), then reproduce the case with a dirs argument pointing to a non-existent directory and a library available in the system lib dir. Done means the dirs argument no longer falls back to the system lib dir and the documented override behavior is covered by a regression test.

Written by the indexing model from the issue text.