It adds useful functions for tests that use struct timespec.
Checked on x86_64-linux-gnu and i686-linux-gnu.
* support/timespec.h: New file. Provide timespec helper functions
along with macros in the style of those in check.h.
* support/timespec.c: New file. Implement check functions declared
in support/timespec.h.
* support/timespec-add.c: New file from gnulib containing
timespec_add implementation that handles overflow.
* support/timespec-sub.c: New file from gnulib containing
timespec_sub implementation that handles overflow.
* support/README: Mention timespec.h.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
