Extensions makefiles - coverage

Hello.

I was having trouble figuring how to use the coverage targets when
using an extension.
I am using approximatively the layout that was proposed here:
http://www.postgresql.org/message-id/51BB1B6E.2070705@dunslane.net
It looks like everything is hard-coded to take the source and the
gcda, gcno files in the base directory, but these files lay in a src
directory with the proposed layout.

It may be better to base the .gcda file discovery on the OBJS
variables when using PGXS.

Please find attached a small patch that implements this. There is
probably a better way than the redundant rm $(gcda_files) / rm *.gcda
to cleanup the generated files.

With the attached patch, the following targets seem to have the same
behaviour as on the current HEAD, both on the whole tree and on
individual contrib modules:

- coverage-html
- clean
- coverage-clean
- clean-coverage

I noticed that make clean leaves gcda and gcov files on the current
HEAD, and this is no different with the given patch.

I also tested it against several pgxn extensions, and it seems to work fine.