(Posting this question for reference purpose, I'll answer immediately)
How to add header search paths to Xcode? Especially when including with this syntax:
include <myheader.h>
Follow up to Eonil's answer related to project level settings. With the target selected and the Build Settings tab selected, there may be no listing under Search Paths for Header Search Paths. In this case, you can change to "All" from "Basic" in the search bar and Header Search Paths will show up in the Search Paths section.
Though this question has an answer, I resolved it differently when I had the same issue. I had this issue when I copied folders with the option Create Folder references
; then the above solution of adding the folder to the build_path worked.
But when the folder was added using the Create groups for any added folder
option, the headers were picked up automatically.
To use quotes just for completeness.
"/Users/my/work/a project with space"/**
If not recursive, remove the /**
Source: Stackoverflow.com