On Xcode, certain things don't work if the headers
aren't also part of the project, namely .h/.cpp
flipping ("Jump to Next Counterpart" in the navigation
menu) and searching the entire workspace for symbols.
The header files are now collected from the directory
of the target and show up in the XCode project under
"Header files" (per target).
I limited this change to Mac since I cannot test anywhere
else, but it would probably not hurt to do the same thing
on other platforms.
According to forums, the add_executable and add_library
commands of CMake should just silently ignore these files,
but add them to the generated target in the project.
356b124e2b