tools/cmake: always use non-ccache CC and CXX variables

tools/cmake: always use non-ccache CC and CXX variables

cmake is a dependency of ccache, which means it is build before ccache
is available and hence must be build with non-ccache CC and CXX. It
currently works, because the cmake build system splits the compiler
variable and treats them as multiple compilers to check.

For "ccache gcc" it first tests for "ccache", which always fails,
because ccache is not a compiler by itself, even if it is available, and
then ends up calling "gcc" alone, effectively never using ccache.

Let's make this explicit by forcing the use of non-ccache CC and CXX.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>