ENH: add DragonFly BSD, which is very close to FreeBSD (#4500)

Alex

Modules/Platform/DragonFly.cmake

@@ -0,0 +1,5 @@
+# DragonFly BSD was forked from FreeBSD and is still very close to it
+# http://www.dragonflybsd.org
+# see http://archive.netbsd.se/?ml=dfbsd-users&a=2007-07&m=4678361
+
+INCLUDE(Platform/FreeBSD)