Thank you for your comments. Unfortunately I won't be able to implement these changes because of time constraints in the current project, sorry if I've wasted your time.
On the bright side I feel like I've learned something so thank you for that. I hope I can make a proper contribution in the future.

@Anastasia I feel like I am following the guidance you quoted here. The defines I've deleted in opencl-c-base.h are blocking the possibility of -cl-ext working and would also get in the way of undefine preprocessor symbols working.
If there is a problem with the additions to OpenCLExtensions.def then we'll need to rework how InitializeOpenCLFeatureTestMacros in initPreprocessor.cpp works.