lilypond.pot broken (SCM missing)

[Michael Käppler]

Hi all,
while adding gettext calls to all warning and error messages I noticed
that the strings from all SCM files are missing in lilypond.pot.

Bisecting yielded that

commit 5e092ee0d9b84d1948dc90e95488e8c9b58576d1
Author: Han-Wen Nienhuys <hanwen@lilypond.org>
Date:   Sat Mar 21 23:46:14 2020 +0100

    Inline scm stepmake templates

caused the regression.

It seems that the definition of $(SCM_FILES) in scm/GNUmakefile does not
reach stepmake/stepmake/po-targets.make,
where it would be needed to run xgettext on it.

I do not know how to fix this correctly, any advice or MR appreciated.

Cheers,
Michael