mk/make-config.sh: inc repro-build (Jelle van der Waa, Arnout Engelen)..

mk/make-config.sh: inc repro-build (Jelle van der Waa, Arnout Engelen)..

Jelle van der Waa reported that ArchLinux reproducible-builds.org
fail because of $MAKEFLAGS using -j32 in the one and -j31 in the
second run.  I responded that our configuration system is make
based, uses make command line arguments, and therefore baking
MAKEFLAGS into the binary is of value for us, just like other
packages bake in ./configure options.

So on Friday the 13th (huh!!!) Arnout Engelen asked

  Would there be any way keep only the 'application-level' options
  from the MAKEFLAGS but leave the 'build-level' options out?

and so this changeset splits $MAKEFLAGS into the portion before
and after the --, baking in only the latter.  This likely is
a GNUism (Solaris using distributed make, likely), but since it
will reenable reproduceability of our MUA for in practice all
Linux distributions, i see no other way except replacing our build
system.