I see this fixed by commit
commit f448ded3a79dccfc1a95f032aa80ab228c2c7a09 (HEAD -> master,
pgrpms/master)
Author: John K. Harvey <john(dot)harvey(at)crunchydata(dot)com>
Date: Wed Aug 15 15:32:56 2018 -0400
Fix broken PGXS=1 builds in -devel package (missing pg_config.h error)
so thanks. Unclear if you noticed this independently or from my mail, but
either way, thanks.