pgsql: Improve support for building PGXS modules with VPATH.

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6697aa2bc25c83b88d6165340348a31328c35de6

Modified Files
--------------
src/Makefile.global.in |   12 ++++++++++-
src/makefiles/pgxs.mk  |   56 ++++++++++++++++++++++++++++++++----------------
2 files changed, 49 insertions(+), 19 deletions(-)