os/osinit.c, os/utils.c: Exclude sigaction code when building for MinGW

MinGW doesn't have sigaction, so this patch is needed for building.

No attempt is made to actually install the fatal error signal handler, as MinGW
will simply terminate the process rather than deliver a fatal signal.

Also avoid using strsignal

Signed-off-by: Ryan Pavlik <rpavlik@iastate.edu>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Tested-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Reviewed-by: Keith Packard <keithp@keithp.com>

os/utils.c

@@ -212,6 +212,9 @@ sig_atomic_t inSignalContext = FALSE;
 OsSigHandlerPtr
 OsSignal(int sig, OsSigHandlerPtr handler)
 {
+#if defined(WIN32) && !defined(__CYGWIN__)
+    return signal(sig, handler);
+#else
     struct sigaction act, oact;
 
     sigemptyset(&act.sa_mask);
@@ -222,6 +225,7 @@ OsSignal(int sig, OsSigHandlerPtr handler)
     if (sigaction(sig, &act, &oact))
         perror("sigaction");
     return oact.sa_handler;
+#endif
 }
 
 /*