Fix compilation by digimars.mak (DMC)

Fix compilation by digimars.mak (DMC)

Also, the script now builds test_cpp executable.

* digimars.mak (DEFINES): Remove -D NDEBUG; replace -D WIN32_THREADS to
-D GC_THREADS.
* digimars.mak (CFLAGS): Add -I libatomic_ops\src.
* digimars.mak (targets): Add test_cpp.exe.
* digimars.mak (gc.dll, gctest.exe): Replace sc with $(CC).
* digimars.mak (gc.def): Refine DESCRIPTION.
* digimars.mak (clean): Also delete gc.dll, gc.lib, gc.map, gctest.map,
test_cpp.map, tests\test.obj, gctest.exe, tests\test_cpp.obj,
test_cpp.exe.
* digimars.mak (test_cpp.exe, tests\test_cpp.obj): New rule.
* digimars.mak (tests\test.obj): Replace compiler options with $(CFLAGS).