Introducing some "global" build out-of-source variable

[Mojca Miklavec]

I would like to propose introducing some "build-out-of-source"
variable that would work for any build system (CMake, meson,
configure, ...).

At the moment we do this semi-optionally for CMake, by default for
meson and don't explicitely support it for other build systems at all
(there might be exceptions, I'm not sure).

At the moment I'm working on an existing port which uses qmake and
implements the workaround manually again. I could add the variable to
the PortGroup, but I would like to see that variable/setting
universally available.

Any thoughts?

Thank you,
    Mojca