PTHREAD_MUTEX_INITIALIZER and PTHREAD_COND_INITIALIZER should only
be used for statically allocated mutexes - not doing it here breaks
building on 64-bit Android.
The play-audio binary links to libOpenSLES, which links indirectly
to libskia. As libskia links to both libpng and libjpeg this means
problems if either of those libraries are installed from a Termux
package into LD_LIBRARY_PATH.
Avoid this by placing the play-audio binary into libexec/, and let
bin/play-audio be a wrapper script which resets LD_LIBRARY_PATH
before calling the binary in libexec/. Fixes#103.
Fredrik Fornwall
Francisco Demartino