configure: add support for neon intrinsics
aarch64: NEON intrinsics dct_unquantize_h263.

Intrinsics only used on aarch64 since the existing ARMv7 NEON asm
is slightly faster (Cortex-A9, gcc-4.8, micro-benchmarks and full
decoding time).

Signed-off-by: James Yu <james.yu@linaro.org>
Signed-off-by: Janne Grunau <janne-libav@jannau.net>