Saves comitters' daily life by avoid #include-ing everything from internal.h to make each file do so instead. This would significantly speed up incremental builds. We take the following inclusion order in this changeset: 1. "ruby/config.h", where _GNU_SOURCE is defined (must be the very first thing among everything). 2. RUBY_EXTCONF_H if any. 3. Standard C headers, sorted alphabetically. 4. Other system headers, maybe guarded by #ifdef 5. Everything else, sorted alphabetically. Exceptions are those win32-related headers, which tend not be self- containing (headers have inclusion order dependencies).