The function strtok_r() is part of the POSIX.1 specification, but is not
available on some platforms. We provide an internal implementation, so we
do not need to rely on a platform implementation.
Add aconvert filter to perform sample format, channel layout, and
packing format conversion.
The aconvert code depends on audio conversion code in libavcodec, so
this requires a dependency on libavcodec.
Based on previous work by S.N. Hemanth Meenakshisundaram and Mina Nagy
Zaki, performed for the GSoC 2010 and 2011.
