Added xarg support to prevent fail when compiling kernel bc library.
Fixed 2 tests which should compile host code to use host compiler, instead of kernel code compiler.
Fixed check if kenel clangxx compiler works for compiling vecmathlib and added resolving correct -stdlib= switch.
Added paths to cmake to find llvm headers and libs.
Added clangxx stdlib flag for compiling kernel library.
Fixed review comments.
Merge branch 'pr-various-cmake-fixes' of github.com:elhigu/pocl