- Stop abuse of the aom_util target to expose aom_ports includes
to IDEs.
- Create aom_ports target only on appropriate platforms.
- Otherwise add the include only sources to the libaom target (aka abuse
the libaom target instead of the aom_util target).
BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76
Change-Id: I82a27a0fc2d6ab13ea273726a4777e21ebdbd247