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.
Fix GCC warning:
asrc_abuffer.c: In function ‘init’:
asrc_abuffer.c:258: warning: passing argument 1 of ‘strtok_r’ discards qualifiers from pointer target type
