I\'d like to add ~/include to my include path for all projects while using Xcode, something like setting the environment variable CPLUS_INCLUDE_PATH in
Add the HEADER_SEARCH_PATHS build setting to your Xcode project. This setting takes a space separated list of paths and if the path ends in a double star (**), it will search subdirectories recursively. So to answer for your example, set HEADER_SEARCH_PATHS to:
~/include
If you had a number of paths to include, you would set it to something like:
~/include ~/my_other_includes/** /usr/local/special_frameworks/**