configure.ac: First check for "theoraenc theoradec" and if that failed check for "theora >= 1.0alpha5". The former ap...

Original commit message from CVS:
* configure.ac:
First check for "theoraenc theoradec" and if that failed check
for "theora >= 1.0alpha5". The former appeared in 1.0beta3 and
deprecate the latter. Also linking on Windows fails with just "theora"
and the version check would fail for the release candidates.
Fixes bug #563718.