meson: enable cross compiling
the inital work for this commit was coming from Mark van der Putten.
In order to not have more options for this, the idea came up to use
mesons autodetection using PATH.
If a cross file is specified, the binaries are used from the system,
rather than from the intree. (Which means --cross-file has the
dependency of efl on the buildsystem)
Differential Revision: https://phab.enlightenment.org/D7415