We have two options.
Look at Preferences->Locations->"Custom Paths" in Xcode's preference. A path added here will be a variable which you can add to "Header Search Paths" in project build settings as "$cppheaders", if you saved the custom path with that name.
Set HEADER_SEARCH_PATHS parameter in build settings on project info. I added "${SRCROOT}" here without recursion. This setting works well for most projects.
About 2nd option:
Xcode uses Clang which has GCC compatible command set.
GCC has an option -Idir which adds system header searching paths. And this option is accessible via HEADER_SEARCH_PATHS in Xcode project build setting.
However, path string added to this setting should not contain any whitespace characters because the option will be passed to shell command as is.
But, some OS X users (like me) may put their projects on path including whitespace which should be escaped. You can escape it like /Users/my/work/a\ project\ with\ space if you input it manually. You also can escape them with quotes to use environment variable like "${SRCROOT}".
Or just use . to indicate current directory. I saw this trick on Webkit's source code, but I am not sure that current directory will be set to project directory when building it.
The ${SRCROOT} is predefined value by Xcode. This means source directory. You can find more values in Reference document.
PS. Actually you don't have to use braces {}. I get same result with $SRCROOT. If you know the difference, please let me know.