I belatedly came across the news regarding warp, a C and C++ preprocessor written by Walter Bright in a joint project with Facebook. This has been released under the Boost license and is available at https://github.com/facebook/warp.

The Facebook blog makes the following statement with regards to  improved efficiency over the GCC preprocessor

Replacing gcc’s preprocessor with warp has led to significant improvements of our end-to-end build times (including linking). Depending on a variety of circumstances, we measured debug build speed improvements ranging from 10% all the way to 40%, all in complex projects with massive codebases and many dependencies