Merge pull request #12558 (GI-shlibpaths)

So far only .la files get correctly converted to absolute paths in the
GIR file. However if there are .so files which depend on a particular
library using GI, they still get only the basename of the .so file.

This improves on the existing absolute_shlib_path.patch not only
figuring out the absolute path of .so files but also falling back on the
absolute path of $out/lib (or $lib/lib with multiple outputs) of the
current build.

With this, we should no longer need to resort to setting LD_LIBRARY_PATH
for all programs that use GI libraries.

I'm merging this because after more than a month no issues came up so
far.