Makefile.in: add source and build directories to C include path, too
ParallelGravity.h: made liveViz.h an angled #include

Without this, Charm's dependency-generation was listing "liveViz.h" --
without path -- as a dependency for any object build from a source that
includes ParallelGravity.h.  Since liveViz.h is found via path search
(and is not kept in the local directory), this is the right include style
to use for it anyway.