Use preprocessor in traditional mode

[dmikushin]

Fortran compiler is supposed to use traditional C preprocessor only. Therefore, we can't use modern C preprocessor features, such as catenation or stringizing

[bryanpkc]

gfortran's preprocessor does not support stringization. There may be a historical reason for that, but we should be able to provide improved preprocessing functionality. LLVM Flang's preprocessor supports stringization (see https://github.com/llvm/llvm-project/blob/main/flang/test/Preprocessing/pp045.F, https://github.com/llvm/llvm-project/blob/main/flang/test/Preprocessing/pp131.F), so I don't think we need to change anything in Classic Flang.

[bryanpkc]

This PR will not be accepted as-is, so closing it for now.