support: Add timespec.h

support: Add timespec.h

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>