Remove support for old versions of timelib in dateinterval.h
Summary: We already unconditionally use the timelib bundled in hhvm-third-party, and that version is new enough to have these things, so there's no need for these compatibility macros.
This also moves a couple of C++ includes that were being done inside an `extern "C"` block, which, if they hadn't already been included by something else, definitely would not have worked.
Closes https://github.com/facebook/hhvm/pull/6309
Reviewed By: @jwatzman
Differential Revision:
D2485491
fb-gh-sync-id:
2817b9a12649ecf95866bd4caada0adba5c979cd