Fixes thread sanitizer warnings.

This attribute shall be used only for IsInside() to make this safe
against a race condition described in #188:

> There is no requirement on the implementation that the ID of the
> created thread be available before the newly created thread starts
> executing.

http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_create.html):

This means that on some pthread implementations (e.g. Haiku), the
assert(thread.IsInside()) could fail.

Closes #188

This method is only used inside assert().

Use the "==" operator instead of pthread_equal().

This allows us to eliminate two boolean flags which are there to avoid
race conditions, and made the thing so fragile that I got tons of
(correct) thread sanitizer warnings.

This completes the bug fix commit
2065e329; if we clear "queued" then we
must clear "queued_song" as well, or another variant of the assertion
fails.

MPD doesn't use this feature anyway.

This introduces a the new class MakeProject, which is used as a base
class for all Makefile based thirdparty libraries.

On OS X, the "libtoolize" command is some Apple tool. The libtoolize
we want is named "glibtoolize" in Homebrew.

.. and make all library symbols hidden by default.

Saves big amounts of .text section size with --gc-sections, because
only this allows discarding unused functions from those (static)
third-party libraries.

In current Android SDK releases, Ant support was removed. Move the
necessary build steps from the former Ant build system to our Makefile,
and call the required build tools from the Android SDK (aapt and dx),
Java SDK (javac) and Info-ZIP (zip) directly.

[mk: copied from Felix's commit
e52b906dba971a1173f9e8f83d32b52ee9f89af3 in the XCSoar project)

Wohooooo, the method Filter::Reset() has been broken because no
implementation of it has ever been called for a loooong time.
And nobody ever noticed it.  WTF.

Fixes another crash bug caused by commit
64d141f7

Fixes crash bug caused by commit
64d141f7

Caused by commit 64d141f7

This wasn't a serious memory leak, because after a mount failure, MPD
would abort anyway, which is subject to the next commit.

This source file isn't compiled when the database is disabled.