FindSquish: Make squish_add_test work with any Squish version
Update the documentation (Squish 4 is from 2010, so people are likely using something newer) and let squish_add_test call either the v3 or v4 macro based on the detected Squish version.
This cannot break things, since mixing incompatible versions would not have worked before.
Topic-rename: FindSquish-add_test
Edited by Brad King