Revert "Remove RAWCPP_FLAGS"

This reverts commit 460eebec65811c6a7bbe11645df322dda868e80d.

Thomas requested to revert the commit with the words:

> Please revert this commit, it is horribly wrong. I'll have a proper look
> later, but not supplying `-traditional` to the C preprocessor is the cause
> of #9828.

the reverted commit was related to #9094