libfdk-aacenc: Enable 7.1 channel encoding

7.1(wide) and 7.1(wide-side) channel layouts are supported in
fdk-aac since the 0.1.3 release.

The earlier versions of fdk-aac didn't include any library
version defines in the public headers, thus checking for
the AACENCODER_LIB_VL0 define is enough to know that we're
building against a new enough version of fdk-aac.

This change includes contributions by Tim Walker,
Michael Niedermayer and Timothy Gu.

Signed-off-by: Martin Storsjö <martin@martin.st>
