Most modules (e.g. gtk+) can be built in a directory separated from the
source directory. Update the scripts that generate .defs and doc.xml files
to handle that. See !11.
The environment variable JHBUILD_SOURCES is not used any more.
Instead the environment variables GMMPROC_GEN_SOURCE_DIR and
GMMPROC_GEN_BUILD_DIR are read. See comments in init_generate.sh.