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.

Convert the fill_buffer to a class method, to allow accessing the private method.

Also remove the leftover Wrapper stuff.

This fixes a build issue on Haiku as it does have sin_len.
Tested on Linux as well.

For some reason AC_CHECK_MEMBER doesn't generate the proper define
in config.h.in, so I used AC_CHECK_MEMBERS.

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)

All it does is "sometimes" close the mixer, but that is also done in
the destructor.