cmSystemTools: Fix compilation on DragonFly BSD

Define `_XOPEN_SOURCE >= 700` to make `gettimeofday()` et al. available.

Source/cmSystemTools.cxx

@@ -6,7 +6,8 @@
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _POSIX_C_SOURCE 200809L
 #endif
-#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__QNX__)
+#if defined(__FreeBSD__) || defined(__DragonFly__) || defined(__NetBSD__) ||  \
+  defined(__QNX__)
 // For isascii
 // NOLINTNEXTLINE(bugprone-reserved-identifier)
 #  define _XOPEN_SOURCE 700