What is the location assumed for C include files and C libraries for Him?
I suspect the latest upgrade of Xcode to have stored the directories in another location, making Nim unusable on my Mac.
You can also add --cincludes:SOMEPATH to your nim c compile line and see if that fixes your problem (if you have some clue what a correct SOMEPATH might be).
I don't know about Xcode, but most C compilers you can add a -H flag to see #include (pre)processing.