BUILD: makefile: include EXTRA_MAKE in the .build_opts construction

BUILD: makefile: include EXTRA_MAKE in the .build_opts construction

EXTRA_MAKE allows to source an external makefile to bring new options
that will result in including add-ons etc. It must be part of the
construction of .build_opts that decides whether or not existing .o
are reusable or need to be rebuilt, otherwise we can end up with a mix
of .o built with some options and others with different options.

No backport is needed, as this appeared in 3.4.