getenv is a very basic function that has been in stdlib.h since
ISO/IEC 9899:1990.  There is absolutely zero need for us to redeclare.

pty.c already includes stdlib.h out of the box so we need nothing.

[Misc #18891]

Passing the optional env hash to PTY.spawn has been supported for years, but it's never documented.
More info: https://bugs.ruby-lang.org/issues/12312

IA64 support was dropped in ticket #15894, so we can drop support for
HP-UX.

This reverts commit 478716f49a19cdd86f629c6a0673c1ff53630c96
partially -- "Removed unnecessary parentheses" part -- because of
a lambda, see 9e25eb308d4fae9a10e120c2b4601916cc38336c.

* Adjusted indentation in license section, and used "(c)"
* Commented out invalid syntax to enable highlighting
* Removed unnecessary parentheses

For a regular block, accepting two arguments is fine as the array
will be autosplatted.   However, a lambda that accepts two arguments
will not work.

We could change the implementation to yield two arguments instead
of an array with a single argument, but that would be less backwards
compatible.

I'm only changing the call-seq to be precise, other examples pass
a literal block that accepts two arguments, and I left those alone
as that will be the most common usage.

Fixes [Bug #17094]

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).