[rfa/testsuite] avoid 'gamma' function name

[Michael Elizabeth Chastain]

> Sounds reasonable to me.  I'm 99.9% sure you're right about the GCC
> bug, too - at least if gcc HEAD defaults to gnu99 mode?

gcc HEAD -std=c99 compiles my test program with 'gamma'
just fine.  So does gcc HEAD -std=c89, for that matter.

So it looks like the particular name 'gamma' is a gnu extension
and is enabled by default.  Bit my ankle, but easy to deal with.

Michael C