Request for slightly more portability - specifically c99 or c11 support

[rsbeckerca]

I have been trying to get clar to work in my environment, which is a HPE NonStop x86 (Big Endian). The compilers available are c99 and c11. gcc is not available nor is likely to be in this decade. This would mean that being able to import CFLAGS, specifically to bypass non-portable flags like -Wall is crucial to being able to run the available compilers.

I am happy to do whatever changes are needed, but would appreciate a bit of direction on how you would like this done.