Use CMAKE_{C,CXX}_COMPILER_LAUNCHER for ccache
Use CMAKE_{C,CXX}_COMPILER_LAUNCHER for ccache

CMake 3.4 and newer which is the required minimum version for LLVM
supports CMAKE_{C,CXX}_COMPILER_LAUNCHER for settting the compiler
launcher such as ccache which doesn't require shifting arguments.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust