For consistency, and because clang-tidy can't figure out if a define is an
include guard unless it ends the file.

Not all of these actually show up in the output, but it's unnecessary to use
these in Markdown anyway.

These caused warnings in compilers where __GNUC__ is not defined.  That could
be fixed, but it makes things quite ugly, these versions are quite old at this
point, and these features are just for developers.  So, instead, simply check
the major version instead of the specific minor version to keep things terse.