The include guard symbol is `FORTRAN_$dir_$file_H_` where $dir is the
subdirectory of `lib` and $file is the basename of the header file.
Those names are mapped to uppercase and hyphens are replaced by
underscores.
Original-commit: flang-compiler/f18@ac9c0e7106
Reviewed-on: https://github.com/flang-compiler/f18/pull/75
4008e3e5d7