Make PostgreSQL version requirement explicit in Makefile and README

The "ifeq (...) $(error ...) endif" works in GNU Make 3.78+. Although, PostgreSQL 9.1+ require GNU Make 3.80+. GNU Make 3.78 was released in 1999 and 3.80 was released in 2002.

Also added a note to the README file which mentions that cstore_fdw supports only PostgreSQL 9.3 and 9.4.

Tested this with different GNU Make versions and PostgreSQL 8.4+.