Update version number check in Makefile for v10+

The IEEE-compliant "expr" command only supports integer comparison, and thus the comparisons here were using string comparison. This worked fine through PostgreSQL version 9.6 but no longer worked when PostgreSQL 10 was released.

This change adds a dependency on bc in order to do a floating point comparison while staying as readable as possible and changing as little as possible. This seems a safe dependency; it's widely available and I suspect it will be present on most operating systems where PostgreSQL is supported (if not all).