GCC 6 defaults to C++14 (or more precisely, gnu++14) instead of C++98
(or gnu++98).

The fdk-aac source doesn't support being built in this mode at the
moment, since it relies on narrowing conversion from unsigned 32 bit
integers to FIXP_DBL (which is a signed data type of the same size).

The same approach is used upstream in Android as well, since
d52f3747.

This fixes buliding with GCC 6.