(almost) fix distcheck

This series of build changes gets distcheck to pass if I disable the actual tests, since some of them fail on my system.

I'm kind of on the fence about which CLEAN target the BUILT_SOURCES should be in.  The examples in the documentation at http://www.gnu.org/software/automake/manual/html_node/Built-Sources-Example.html use CLEANFILES, but the built sources are included in the dist tarball so if you just extract it and build, it doesn't have to build the BUILT_SOURCES files, but if you clean and build again, it does.  I don't think anyone is really going to care, though so I just stuck with CLEANFILES.

Commit c31c775 is a little weird, but trying to leapfrog src/GLdispatch/mapi to recurse into src/GLdispatch/mapi/vnd-glapi makes it really hard to include the files vnd-glapi needs from mapi in the dist tarball.  Just adding it to src/GLdispatch's EXTRA_DIST rule makes it include src/GLdispatch/mapi/vnd-glapi/Makefile, which is wrong.  Let me know if swapping these directories like this doesn't make sense and I can try to come up with something else.