Use _setjmp()/_longjmp() only on BSD-like systems for now.

compiler/condsyms.nim

@@ -136,5 +136,7 @@ proc initDefines*() =
   declareSymbol("emulatedthreadvars")
   if platform.OS[targetOS].props.contains(ospLacksThreadVars):
     defineSymbol("emulatedthreadvars")
-  if isDefined("posix"):
+  case targetOS
+  of osSolaris, osNetbsd, osFreebsd, osOpenbsd, osMacosx:
     defineSymbol("nimRawSetjmp")
+  else: discard