codegen/generate_defs_and_docs.sh: Update for non-source-dir builds
codegen/generate_defs_and_docs.sh: Update for non-source-dir builds

Most modules (e.g. atk) can be built in a directory separated from the
source directory. Update the script that generates .defs and doc.xml files
to handle that.
The environment variables GMMPROC_GEN_SOURCE_DIR and GMMPROC_GEN_BUILD_DIR
are read. See comments in init_generate.sh.

GNOME/atkmm,GNOME/atkmm,GNOME/atkmm,GNOME/atkmm