".jhbuildrc" -> ".config/jhbuildrc" in .install

Recommended location for this file is now ~/.config/jhbuildrc
(~/.jhbuildrc still works).
Don't install python.pc symlink

This apparently isn't necessary anymore.
Only pass PYTHON=... to gobject-introspection

PYTHON=/usr/bin/python2 is only needed when building
gobject_introspection; other modules are fine without it, and several
actually won't build if it's passed.
pkgrel bump 8 -> 9