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).
It is more conventional to use compiler to pre-process and
assemble the `.S` file rather than forcing Makefile to use `.s`.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65952 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
