[Proposal] Generate pkg-config for server module development

Hello hackers!

I've been looking into making it easier for extension authors to use
other build
systems than make. I don't have everything in place just yet, but I
believe this
first part where we generate pkg-config files for building server
modules can be
useful on it's own.

I'm not entirely sure about what parameters we should include in the
pkg-config
file, but what I have in the attached patch is inspired by PGXS and the
pkg-config files we already generate for shared libraries.

A meson.build file using this could look something like this:

    project('my_extension', 'c', default_options: { 'b_lundef': false })
    pg_dep = dependency('postgresql-server')
    shared_library(
        'my_extension',
        name_prefix: '',
        sources: ['src/my_code.c'],
        dependencies: pg_dep,
        install: true,
        install_dir: pg_dep.get_variable('libdir'),
    )

I choose the Makefile to add this to quite arbitrarily, I'm sure
arguments can
be made for many of them.

Any feedback is highly appreciated! We would also need to add pkg-config for
building frontend tools I believe.

--
Anders Åstrand
Percona