introduce cpp.compilerPathByLanguage

The setup-toolchains tool stores the compiler path per language in
compilerPathByLanguage for toolchains that provide different compiler
binaries for different languages.  For the gcc/clang toolchains we now
use g++/clang++ for C++ and gcc/clang for everything else.

Change-Id: I62ce5a5a4217e2832fafd03301ff75efab62a608
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
work around bug in icecc client

When passing the -x argument to gcc, current icecc versions (at least
until 1.0.1) refuse to distribute the job to a remote machine.
We work around this issue by skipping the -x argument if the file tag
matches the language determined by the file extension.

Task-number: QBS-625
Change-Id: Id12a158b8adf202ddc771f411e9539e26a5598be
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>