Make optional backends respect setup.cfg

Several optional backends were not checking the setup.cfg before testing if they can be installed (which raised and broke the build for some, see #1949). For more info see comment in https://github.com/matplotlib/matplotlib/pull/1949#issuecomment-23132249.

~~This feels pretty ugly. I think it should be handled at the super class OptionalBackendPackage so it can't be overlooked when a new backend gets added. However it is a temporary fix.~~ Implemented in super class now.