bpo-29643: Fix check for --enable-optimizations (GH-873)

The presence of the ``--enable-optimizations`` flag is indicated by the
value of ``$enableval``, but the configure script was checking ``$withval``,
resulting in the ``--enable-optimizations`` flag being effectively ignored.
(cherry picked from commit 8cea5929f52801b0ce5928b46ef836e99a24321a)
3 files changed