In a couple of places we said "not supported on this platform" for cases
that aren't really platform-specific, but could depend on configuration
options such as --with-openssl.  Use "not supported by this build" instead,
as that doesn't convey the impression that you can't fix it without moving
to another OS; that's also more consistent with the wording used for an
identical error case in guc.c.

No back-patch, as the clarity gain is small enough to not be worth
burdening translators with back-branch changes.