installing "ruby" platform gem should either use the "pkg-config" gem or omit mentioning it in error messages

[flavorjones]

Currently (v1.5.0+) if a system does not have the pkg-config utility installed, the user will see an error message like this:

Could not configure the build properly (pkg_config). Please install either the pkg-config utility or the pkg-config rubygem.

Unfortunately, the extconf.rb does not actually use the pkg-config gem if it's present. We should either add that support to the extconf.rb file or remove mention of the gem from the error message.