Improve support for building PGXS modules with VPATH.

A VPATH build will be performed when the module's make file path is not
the current directory or when USE_VPATH is set.

This will assist packagers and others who prefer to build without
polluting the source directories.

There is still a bit of work to do here, notably documentation, but it's
probably a good idea to commit what we have so far and let people test
it out on their modules.

Cédric Villemain, with an addition from me.