Make g-ir-compiler find files installed by make install

Make g-ir-compiler find files installed by make install

When ./configure --prefix $HOME/some/where is used gobject-introspection
will happily install the files into $HOME/some/where/data/gir-1.0 but
it will refuse to find them. Apply the same trick as in
girepository/girepository.c:init_globals to find the gir files.

Unifiy the name gir-1.0 in GIR_SUFFIX and use it throughout the
project, introduce GIR_DIR which holds the path to the gir files and
update girparser and transformer.py to look into this path.