Use PGXS builds on 9.2+

[schmiddy]

The installation instructions say that plpgsql_lint must be placed into PostgreSQL's ./contrib/ directory. In fact, this is not necessary, at least not for versions 9.2+ -- the PGXS build steps work fine on these recent versions. AFAICT, versions 9.1 or older must still be built inside ./contrib/ though, because this bit of the Makefile:

override CFLAGS += -I$(top_builddir)/src/pl/plpgsql/src

is needed to pick up plpgsql.h from the source directory on these old versions.

I think the Makefile could look at MAJORVERSION and only insist on being built inside of ./contrib/ if it's being built against 9.1 or older.