cmake: detect clang compiler anf fix some link issues
ipfilter: remove two unused functions
ipfilter: remove two unreferenced functions
intra: remove an unreferenced function
pixel: remove vector class headers from pixel-ssse3.cpp
vector: bypass a number of functions we do not use when compiled by clang

The sooner these classes go away the better
blockcopy: isolate vector class routines together (firing squad)
cmake: enable clang to compile intrinsic primitives without version checks