liboom

Add an m4 macro to include liboom in packages, configure/build it if needed but not install it, as a "make check" dependency only.
- Get inspiration from libopts
- Get inspiration from https://neverpanic.de/blog/2014/04/10/autoconf-ac-config-subdirs-with-custom-flags-for-subprojects/ and https://trac.macports.org/browser/trunk/base/aclocal.m4?rev=118736&marks=99-307#L95
I still need to make it embeddable in projects source distribution files. If embedded, be careful with
recursively builds. It is mostly designed to be a unit-test dependency, a convenience library, not a
project package dependency. Build it only where needed (probably for unit tests) and not to install it
with the project.