[HACKERS] building libpq.a static library

I maintain static libraries for libpq for the R programming language
(we need static linking to ship with the binary packages).
Unfortunately currently the standard postgres makefile only generates
a shared library for libpq, not a static one.

In order to make a static library I always manually edit
./src/interfaces/libpq/Makefile and add a target:

libpq.a: $(OBJS) ar rcs $@ $^

And then run 'make libpq.a' in the libpq directory.

This works but it's a bit of a pain to maintain. I was wondering if
this hack could be merged so that the standard 'configure
--enable-static' script would install a static library for libpq
alongside the shared one.