Enough is enough, C99 is not too much to ask in 2017

Supports multiple indexes now, to be used for non-tunable 'parameters'

Instead of listing all prerequisites multiple rules were added for the
same target, probably while doing a merge

Actually, the complex cases have not been checked so far.
Furthermore, there has been an inconsistency between setting

set("gpu",1)

and *NOT* setting a GPU kernel via the set mechanism. Then, the
default kernel (which is *NOT* GPU) has been used