Fix pcntl_rfork build for DragonFlyBSD

RFTSIGZMB flag unsupported, guarding with the said flag instead of system
in case the implementaion catches up with FreeBSD's.

Closes GH-7918.

Author: devnexen

NEWS

@@ -9,6 +9,9 @@ PHP                                                                        NEWS
 - FPM:
   . Fixed memory leak on invalid port. (David Carlier)
 
+- pcntl:
+  . Fixed pcntl_rfork build for DragonFlyBSD. (David Carlier)
+
 06 Jan 2022, PHP 8.1.2RC1
 
 - Core:

ext/pcntl/pcntl.c

@@ -1528,9 +1528,11 @@ PHP_FUNCTION(pcntl_rfork)
 		flags |= RFPROC;
 	}
 
+#ifdef RFTSIGZMB
 	if ((flags & RFTSIGZMB) != 0) {
 		flags |= RFTSIGFLAGS(csignal);
 	}
+#endif
 
 	pid = rfork(flags);