pg_buildext's use of VPATH interferes with the setting in hstore/Makefile,
hard-code the value here.

--- a/hstore/Makefile
+++ b/hstore/Makefile
@@ -49,7 +49,7 @@ VPATH := $(dir $(firstword $(MAKEFILE_LI
 endif
 endif
 
-mklibdir := $(if $(VPATH),$(VPATH)/../tools,../tools)
+mklibdir := $(CURDIR)/../tools
 include $(mklibdir)/numeric.mk
 
 PGXS := $(shell $(PG_CONFIG) --pgxs)
