Enable CPU profiling on Cygwin

* src/syssignal.h [CYGWIN] (PROFILER_CPU_SUPPORT): Revert previous
change that undefined this.
(SIGEV_SIGNAL): Ensure that this is defined as a macro.
* src/profiler.c [CYGWIN] (timer_getoverrun): Define as a macro on
  Cygwin.

src/profiler.c

@@ -217,6 +217,12 @@ static EMACS_INT current_sampling_interval;
 
 /* Signal handler for sampling profiler.  */
 
+/* timer_getoverrun is not implemented on Cygwin, but the following
+   seems to be good enough for profiling. */
+#ifdef CYGWIN
+#define timer_getoverrun(x) 0
+#endif
+
 static void
 handle_profiler_signal (int signal)
 {

src/syssignal.h

@@ -32,13 +32,17 @@ extern void unblock_tty_out_signal (sigset_t const *);
 #define FORWARD_SIGNAL_TO_MAIN_THREAD
 #endif
 
+/* On Cygwin as of 2015-06-22 SIGEV_SIGNAL is defined as an enum
+   constant but not as a macro. */
+#if defined CYGWIN && !defined SIGEV_SIGNAL
+#define SIGEV_SIGNAL SIGEV_SIGNAL
+#endif
+
 #if defined HAVE_TIMER_SETTIME && defined SIGEV_SIGNAL
 # define HAVE_ITIMERSPEC
 #endif
 
-/* On Cygwin, setitimer does not support ITIMER_PROF, so we can't
-   support CPU profiling. */
-#if (defined SIGPROF && !defined PROFILING && !defined CYGWIN \
+#if (defined SIGPROF && !defined PROFILING \
      && (defined HAVE_SETITIMER || defined HAVE_ITIMERSPEC))
 # define PROFILER_CPU_SUPPORT
 #endif