cmake: detect and handle Mac OS X 10.8 default compiler

It can't build any of the vector primitives, but at least now it generates
a working x265 exectuable

--HG--
extra : amend_source : 643b84cba6a3a4014d38d67a8d507a97fb7e1190
replace 64s with MAX_CU_SIZE
motion: split mvcost from subpelCompare
TComPrediction: simplify xPredInterLumaBlk
TComPrediction: pre-allocate intermediates temp buffer