regex(3): Resolve issues with higher WARNS levels

libc is set for WARNS=2, but the incoming libregex will use WARNS=6.
Sprinkle some casts and (void)bc's to alleviate the warnings that come along
with the higher WARNS level.

These 'bc' parameters could be outright removed, but as of right now they
will be used in some parts of libregex land. Silence the warnings instead
rather than flip-flopping.