distinguish between 'defined' and 'declared'

lib/system/widestrs.nim

@@ -10,7 +10,7 @@
 # Nimrod support for C/C++'s `wide strings`:idx:. This is part of the system
 # module! Do not import it directly!
 
-when not defined(NimString):
+when not declared(NimString):
   {.error: "You must not import this module explicitly".}
 
 type
@@ -103,7 +103,7 @@ proc newWideCString*(source: cstring, L: int): WideCString =
 proc newWideCString*(s: cstring): WideCString =
   if s.isNil: return nil
 
-  when not defined(c_strlen):
+  when not declared(c_strlen):
     proc c_strlen(a: cstring): int {.
       header: "<string.h>", noSideEffect, importc: "strlen".}