[cmake] Do not resolve the REALPATH too early.

In some ccache setups we have the compiler soft link to ccache.

Eg. /usr/local/bin/g++ -> /usr/local/bin/ccache. If we resolve the link too
early we will take the wrong branch assuming the compiler was set as
CMAKE_CXX_COMPILER="ccache g++" and try to get the second token of the command.