pg_upgrade: remove C99 compiler req. from commit 3c0471b5fd

pg_upgrade:  remove C99 compiler req. from commit 3c0471b5fd

This commit required support for inline variable definition, which is
not a requirement.

RELEASE NOTE AUTHOR:  the author of commit 3c0471b5fd
(pg_upgrade/tablespaces) was Justin Pryzby, not me.

Reported-by: Andres Freund

Discussion: https://postgr.es/m/20201016001959.h24fkywfubkv2pc5@alap3.anarazel.de

Backpatch-through: 9.5