Make extension name detection dynamic based on META.json.

[decibel]

This is based on what David did at https://github.com/theory/pg-semver/blob/master/Makefile

Note: It would be nice if the 3 other templates built on top of what's in root instead of duplicating everything... is there a reasonable way to do that?

TODO: Make version detection dynamic from META.json as well and create a target that creates the .control file dynamically