motion: use fast weighted subpel refine

Don't do the full-blown weighted motion compensation for ME.  Just interpolate
the weighted full pel pixels. It is not 100% accurate to the pixels that will
be used to encode the final prediction; but close enough for ME.

Testing with sintel_trailer_2k_720p24.y4m at medium preset and all defaults
x265 [info]: 651 of 1124 (57.92%) P frames weighted

before: 1253 frames in 512.74s (2.44 fps), 223.51 kb/s, Global PSNR: 50.552
after:  1253 frames in 410.25s (3.05 fps), 223.59 kb/s, Global PSNR: 50.589
cmake: almost revive Xcode support

# macbrew based instructions
brew install cmake --HEAD
cmake -G Xcode ../source
open x265.xcodeproj

> cmake --version
cmake version 2.8.12.20131121

The static library is still not linking properly, so the cli does not link
as well; but it does build the shared library
cmake: backout cmake_policy(); cmake is idiotic

Why would you issue an error if your version of cmake doesn't know about the
given policy?  Especially if the selected policy is OLD?  Hello??
cpu: fix non-Windows build with ASM disabled