Makefile: error earlier when site.mk is missing

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.

Makefile

@@ -19,8 +19,9 @@ escape = '$(subst ','\'',$(1))'
 GLUON_SITEDIR ?= site
 $(eval $(call mkabspath,GLUON_SITEDIR))
 
-$(GLUON_SITEDIR)/site.mk:
-	$(error No site configuration was found. Please check out a site configuration to $(GLUON_SITEDIR))
+ifeq ($(realpath $(GLUON_SITEDIR)/site.mk),)
+$(error No site configuration was found. Please check out a site configuration to $(GLUON_SITEDIR))
+endif
 
 include $(GLUON_SITEDIR)/site.mk