fix const-correctness of argument to stime

it's unclear what the historical signature for this function was, but
semantically, the argument should be a pointer to const, and this is
what glibc uses. correct programs should not be using this function
anyway, so it's unlikely to matter.

include cleanups: remove unused headers and add feature test macros

depends on settimeofday which needs _GNU_SOURCE feature test

initial check-in, version 0.5.0