ffmpeg plugin: when decoded stream duration is unavailable, attempt fallback to container duration (fix MusicPlayerDaemon/MPD#110)

Closes #107

Workaround for the following LAME build failure:

 error: inlining failed in call to always_inline '_mm_sqrt_ps': target
 specific option mismatch

This is because the LAME build scripts do not check whether SSE is
available; they only check for the presence of the "xmmintrin.h"
header.

Requiring a Pentium 3 CPU is reasonable enough, and it's the first CPU
to feature SSE support.

This fixes an assertion failure caused by resuming playback before the
decoder has finished startup.

.. and not the stack buffer.  This made the AllocatedString destructor
crash.

Closes #52

Was broken by commit 8a86460b

Closes #87

Addresses #82.

Previously, the ffmpeg decoder only checked for the "generic"
MusicBrainz metadata keys used in other metadata container formats.

We will reuse these tags elsewhere.

Enable it in the Windows build script, closes #78.

LAME currently doesn't support Android:

 checking host system type... Invalid configuration `arm-linux-androideabi': system `androideabi' not recognized

This commit is similar to 788e3b31,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

Add ffmpeg decoder support for *.adx files (Sega game console)

Closes #60

Patch from
 https://svnweb.freebsd.org/ports/head/audio/musicpd/files/patch-src_system_ByteOrder.hxx?revision=441921&view=co
(with a tiny modification)

Closes #59

Apparently, this makes old clang versions happy ("'alignas' attribute
cannot be applied to types).