If an error has already been set by f.directory->storage->GetInfo(),
don't set it again.

https://bugs.musicpd.org/view.php?id=4577

It was used in a wrong way, which did not deal with errors
consistently.  And if that's wrong, there is no need for FindStorage()
at all - let's remove it and the confusion around it.

An empty packet would be a command for avcodec_send_packet() to
finalize the codec.

Fixes https://bugs.musicpd.org/view.php?id=4588

By declaring the variable-length array to have a nominal size of 1,
struct TagPoolSlot shrinks from 24 bytes to 16 bytes, because "ref"
and "item" now both fit in one machine word.

The "packed" attribute triggers a clang 4.0 warning, and it's not
necessary.  All we want is correct allocation of this
dynamically-sized struct.

Support for GCC has been dropped from NDK r13.  Let's switch
everything to clang before they really remove GCC completely.

Causes compatibility problems, because FFmpeg expects the GNU as
syntax.

This FFmpeg bug report has some information, but it was closed as
"invalid": https://trac.ffmpeg.org/ticket/5158

For Android NDK r13.

DESTDIR is for installing to a chroot, but this is not one.  Instead,
we want real absolute paths in the pkg-config file.

Fixes various static library linker failures ("bad extended name entry
at header"), because the wrong "ranlib" was used.

autoconf depends on libtoolize.

This doesn't make sense on Android; there is no home directory for
users.

change the _delay API doc so that it matches its implementation behavior.

This further optimize the performance. 

This is now working properly with a conditional variable bug being fixed (8bbfb5cd).

pthread_cond_timedwait() in PosixCond.hxx:timed_wait(PosixMutex...) returns
EINVAL, if ts.tv_nsec >= 1E9. In this case, it returns to early.

Find attached a patch which fixes this. I chose a compare-subtraction method
to keep ts.tv_nsec below 1E9.

Another option would be
ts.tv_sec += ts.tv_nsec / 1000000000;
ts.tv_nsec %= 1000000000;

But I guess this takes more time on some ARM processors, which don't support
hardware division.