convert regex.c, strftime.c mktime.c to standard C

[Dan Nicolaescu]

These files currently get -Wold-function-definition warnings.
They were not converted in the first place because they are imported
from other projects.

The last two come from gnulib, but gnulib is so different now that
updating those files is not feasible, gnulib support would need to be
redone from scratch.

Not sure how close regex.c is to upstream, but the extra differences
from converting to standard C won't be too bad.