Avoid the use of for loop declarations.

This C99 syntax is now almost entirely contained to test programs;
at this point using such syntax is just an obstacle to a successful
build rather than anything useful.

It is now possible to build with gcc in C89 mode, and probably some
other pre-C99 compilers.

One annoyance is the reference code for the RNG uses such declarations.
We avoid changing the reference code and instead arrange for the test
to be skipped unless compiler support is available.