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
d52f374768.
This fixes buliding with GCC 6.
While the source is C++, it doesn't use anything from the C++
runtime, so by building with -fno-exceptions -fno-rtti, the
dynamic library doesn't have to be linked to libstdc++. This
also simplifies things for users of the static library.