Hi,
When postgresql compiled from source and installed into default dirs pg_config not accessible from PATH. My suggestion below. One can easily change pg_config location in Makefile and there won't be no warnings.
I suggest to modify Makefile:

1. Instead of
   PGXS := $(shell pg_config --pgxs)
   do
   PG_CONFIG = pg_config
   PGXS :=$(shell $(PG_CONFIG) --pgxs)
2. Add OBJS = zson.o. I don't understand why it's removed. Without it i can compile extension but get error when try to install it
   incompatible library "/usr/local/pgsql/lib/zson.so": missing magic block