libnethack pr385

roll parts of pr385 into source tree

This does not take the PR as is.

Unlike the PR, this streamlines and minimizes the integration somewhat:

- use hints/include mechanism instead of creating alternative
  Makefile.dat, Makefile.src, Makefile.top, Makefile.utl in sys/lib;
  those would have been a maintenance nightmare.

- don't have alternative mkmkfile.sh and setup.sh in sys/lib.

- sys/lib/libnethackmain.c differed from sys/unix/unixmain.c by
  very little, so just place a small bit of conditional code at the
  top of sys/unix/unixmain.c instead.

- changed the conditional code bits from __EMSCRIPTEN__ to
  CROSS_TO_WASM.

- You should be able to build the wasm result by:
    cd sys/unix ; sh setup.sh hints/linux.2020 ; cd ../..
    make fetch-lua    (<-one time)
    make WANT_LIBNH all

- You should be able to build LIBNBH by:
    cd sys/unix ; sh setup.sh hints/linux.2020 ; cd ../..
    make fetch-lua    (<-one time)
    make CROSS_TO_WASM=1 all

As it is currently coded, winshim.c requires C99.

sys/unix/unixmain.c

@@ -45,7 +45,17 @@ static void NDECL(wd_message);
 static boolean wiz_error_flag = FALSE;
 static struct passwd *NDECL(get_unix_pw);
 
-int
+#if defined(CROSSCOMPILE_TARGET) && defined(CROSS_TO_WASM)
+/* for cross-compiling to WebAssembly (WASM) */
+#include <emscripten/emscripten.h>
+/* if WebAssembly, export this API and don't optimize it out */
+#define KEEP EMSCRIPTEN_KEEPALIVE
+#else
+#define KEEP
+#endif
+
+
+int KEEP
 main(argc, argv)
 int argc;
 char *argv[];