Avoid that clcache invokes itself when frozen to an .exe file

Windows paths are case insensitive, hence compare them as such to
avoid that we accidentally consider ourselves to be the real compiler.
Avoid type error exception in case clcache cannot tell the source file name

In case the command line analysis indicates that no source file was given,
we would call invokeRealCompiler() without trying to capture the output. This
caused us to return stderr and stdout as 'None', which would fail lateron
when writing to stdout/stderr.