New macro (NO_WINMAIN_ENTRY) to prefer main() instead of WinMain (Win32)

* tests/test.c (WinMain): Do not define if NO_WINMAIN_ENTRY (define
main() instead).
* tests/test_cpp.cc (WinMain): Define (instead of main()) for MSWIN32
and WinCE (but not for MinGW and not if NO_WINMAIN_ENTRY).
huge_test: turn GC_WORD_MAX into a signed constant

* tests/huge_test.c (GC_IGNORE_WARN): Refine comment.
* tests/huge_test.c (GC_WORD_MAX): Replace with GC_SWORD_MAX; change
type to GC_signed_word.