Makes MAGIC_HAT kludge optional.

This adds a build option called 'magic_hat', and makes the code for
MAGIC_HAT_GPS and MAGIC_LINK_GPS conditional on it.  It currently
defaults to True on Linux and False otherwise.  It's meaningless on
non-Linux platforms, though at present there are no
conditionally-present options, so this one is also present on all
platforms.

TESTED:
Tested on a Beaglebone Black with suitable symlinks added (and running
a kernel with PPS_CLIENT_KTIMER turned off.  Verified that the kludge
behaves as intended when enabled, and is inoperative when disabled.

SConstruct

@@ -174,6 +174,8 @@ boolopts = (
     ("leapfetch",     True,  "fetch up-to-date data on leap seconds."),
     ("minimal",       False, "turn off every option not set on the command line"),
     ("timeservice",   False, "time-service configuration"),
+    ("magic_hat", sys.platform.startswith('linux'),
+     "special Linux PPS hack for Raspberry Pi et al"),
     ("xgps",          True,  "include xgps and xgpsspeed."),
     # Test control
     ("slow",          False, "run tests with realistic (slow) delays"),
@@ -845,7 +847,8 @@ else:
         env["pps"] = False
 
     # Simplifies life on hackerboards like the Raspberry Pi
-    confdefs.append('''\
+    if env['magic_hat']:
+        confdefs.append('''\
 /* Magic device which, if present, means to grab a static /dev/pps0 for KPPS */
 #define MAGIC_HAT_GPS	"/dev/ttyAMA0"
 /* Generic device which, if present, means to grab a static /dev/pps0 for KPPS */

gpsmon.c

@@ -1318,6 +1318,7 @@ int main(int argc, char **argv)
 	/* this guard suppresses a warning on Bluetooth devices */
 	if (session.sourcetype == source_rs232 || session.sourcetype == source_usb) {
 	    session.pps_thread.report_hook = pps_report;
+	    #ifdef MAGIC_HAT_ENABLE
 	    /*
 	     * The HAT kludge. If we're using the HAT GPS on a
 	     * Raspberry Pi or a workalike like the ODROIDC2, and
@@ -1326,8 +1327,9 @@ int main(int argc, char **argv)
 	     */
 	    if ((strcmp(session.pps_thread.devicename, MAGIC_HAT_GPS) == 0
 		 || strcmp(session.pps_thread.devicename, MAGIC_LINK_GPS) == 0)
-	    		&& access("/dev/pps0", R_OK | W_OK) == 0)
-		session.pps_thread.devicename = "/dev/pps0";
+	    	 && access("/dev/pps0", R_OK | W_OK) == 0)
+			session.pps_thread.devicename = "/dev/pps0";
+		#endif /* MAGIC_HAT_GPS && MAGIC_LINK_GPS */
 	    pps_thread_activate(&session.pps_thread);
 	}
 #endif /* PPS_ENABLE */

timehint.c

@@ -459,6 +459,7 @@ void ntpshm_link_activate(struct gps_device_t *session)
 	} else {
 	    init_hook(session);
 	    session->pps_thread.report_hook = report_hook;
+	    #ifdef MAGIC_HAT_ENABLE
 	    /*
 	     * The HAT kludge. If we're using the HAT GPS on a
 	     * Raspberry Pi or a workalike like the ODROIDC2, and
@@ -467,8 +468,9 @@ void ntpshm_link_activate(struct gps_device_t *session)
 	     */
 	    if ((strcmp(session->pps_thread.devicename, MAGIC_HAT_GPS) == 0
 		 || strcmp(session->pps_thread.devicename, MAGIC_LINK_GPS) == 0)
-		&& access("/dev/pps0", R_OK | W_OK) == 0)
-		session->pps_thread.devicename = "/dev/pps0";
+		 && access("/dev/pps0", R_OK | W_OK) == 0)
+			session->pps_thread.devicename = "/dev/pps0";
+		#endif /* MAGIC_HAT_GPS && MAGIC_LINK_GPS */
 	    pps_thread_activate(&session->pps_thread);
 	}
     }