Major Makefile.am housecleaning. Passes distcheck.

Move all occurrences of swig_built_sources out of Makefile.am's.
Move all SWIG related use of BUILT_SOURCES out of Makefile.am's.
Clean up 'if PYTHON' conditionalization in gr-*

Still left to do: fix Makefile.swig CLEANFILES and no_dist_files...

Merged eb/gcell-wip2 rev 10130:10152 into trunk.

This makes several gcell related changes.

{{{
The first two are INCOMPATIBLE CHANGES:

(1) The gcell portion of the code base was reorganized. As part of that
reorganization, the paths to the include files changed. They are now...