It only works as the last argument on "apply".  Using quasiquote for this would
be a little over-the-top.

Also, almost all the other build system, even the ones using quasiquote, also
pass them at the end of the argument list.  Just scons-build-system doesn't at
one single place of multiple, and meson-build-system doesn't with
configure-flags.

I'm not opposed to doing it like you suggest, but let's do it for all the
build systems in an extra patch.

Pushed to guix master as commit f51fd97ec54a98668d63c52d8a6bd75d8dc3292e.