Allows advanced compiler optimization.

Most importantly: always disable C++ exceptions.  It was surprising to
see MPD terminate due to an unexpected exception.

The last piece to allow -ffast-math.

Avoid NaN to allow -ffast-math.

Use a boolean flag instead.

Avoid NaN to allow -ffast-math.

Avoid NaN to allow -ffast-math.

Fixes crash.

Restores the features from the previous draft commands "findin" /
"searchin".

This reverts commit a577944a.

Will be replaced by new options for the old commands "search" and
"find".

According to the protocol documentation, matching the file name was
wrong.  This removes some awkward special-case code.

Merge lots of duplicate code.

There hasn't been a maintainer for this plugin for years.  It may even
fail to build.

Fixes the OpenAL detection which was broken because enable_osx was
uesd which was set later.