initdb needs pqsignal() even on Windows.

I had thought we weren't using this version of pqsignal() at all on
Windows, but that's wrong --- initdb is using it (and coping with the
POSIX-ish semantics of bare signal() :-().  So allow the file to be
built in WIN32+FRONTEND case, and add it to the MSVC build logic.

src/include/port.h

@@ -463,11 +463,8 @@ extern int	pg_check_dir(const char *dir);
 extern int	pg_mkdir_p(char *path, int omode);
 
 /* port/pqsignal.c */
-/* On Windows, we can emulate pqsignal in the backend, but not frontend */
-#if !defined(WIN32) || !defined(FRONTEND)
 typedef void (*pqsigfunc) (int signo);
 extern pqsigfunc pqsignal(int signo, pqsigfunc func);
-#endif
 
 /* port/quotes.c */
 extern char *escape_single_quotes_ascii(const char *src);