Log Message:
lang/racket: Add DragonFly support

LDFLAGS.DragonFly+= -rdynamic
Add four patches to support DragonFly:
  * Teach configuration about DragonFly
  * Add DragonFly to BSD signal handler code
  * Fix mbsrtowcs conftest segfault of configure script
  * Fix cast on string.c

Patches contributed by:
  Chris Turner
  Jelle Hermsen

Patches will be submitted upstream.
Revbump due to possible change due to result of mbstrowcs test.