and replace old unused and poorly named FIND_ZIP. The new code prefers
pkgconfig and only falls back to manual detection if needed.
This also prioritizes detected zlib flags and includes to be preferred
over the ones that are added by pkgconfig for libpng, tifflib etc. which
would otherwise have caused the system default zlib to be used.
The python.m4 that ships with automake hardcodes the library path for
platfrom dependent files to `lib`, which is wrong for multiarch systems.
Instead we import the `m4/python.m4` from pygobject, which does not have
this problem and also ensures we have the same behavior as pygobject
which we are depending on for pyvips8.
Due to this change some minor changes to how the `bootstrap.sh` script
clean up the `m4` folder where needed.
The python search order has been modified to prefer the default python
over python2 over python3.
