Warn if the imported build is using ccache as a compiler symlink

ccache can be used with CMake in at least two ways:

1. as -DCOMPILE_C|XX_COMPILER_LAUNCHER=ccache
2. as /usr/lib/ccache/bin/g++

Usually the Kits do not have /usr/lib/ccache/bin/g++ as a compiler, and if the user forces the right Qt kit on the imported location it will cause CMake to recompile everything without ccache