Rules are based on the files' error messages when included directly.
These differ for different compilers, hence separate files for gcc and
clang. Newer compiler versions can include additional header files for
intrinsics, hence version specification in filenames - mostly an issue
for forwards compatibility. Backwards compatibility should work out for
most cases. Rules for backwards compatibility to some gcc versions
included (tested versions listed in the file).
When two possible public headers are suggested, both are allowed
and it's left up to the iwyu standard behaviour which to suggest.
Paul Seyfert