The site.mk target was only evaluated after the whole makefile was
parsed. This caused the GLUON_DEPRECATED error to be emitted first
(hiding the more helpful message that no site config was found) on Gluon
2021.1.x, where GLUON_DEPRECATED is used in a toplevel if in targets.mk.

By moving the check from recipe context to the toplevel, we ensure that
it is evaluated during parsing.