Refactored %s escape into WinSize handler
Copied tests/{signature.h,macros.h} from gnulib and added check target to Makefile

signature.h was modified to explitily use gcc's unused attribute; if we want to
work back in its macro abstraction so that it works with other compilers, that's
fine...and probably a good idea.

The object file of the same name as the test (after the test-* prefix) is
automatically linked. Tests are automatically executed by the `check' target
after having been compiled.
Added winmsgcond test case using gnulib-style testing

Since gnulib is actively maintained and is intended to represent a high quality,
portable library for the GNU project, I figured this would be an excellent
starting point. It is simple to use and we can always migrate to a framework
later if we so choose to. I also like its idea of raising SIGABRT to generate a
core dump, which is far more useful than conventional frameworks' solutions in
other languages. Granted, that also means it aborts immediately without running
all the other tests, which could be a pita.