Clang: never include a define for __cplusplus.

The flags used by the project explorer to interrogate the toolchain for
compiler-defined #defines always indicate a C++ file. So, this will
always include a #define for __cplusplus. When editing C code with
the clang code model, this results in extern "C" linkage specifications,
which result in warnings or errors.

The proper fix is described in QTCREATORBUG-11709, but is too big to be
done within the 3.1 timeframe.

Task-number: QTCREATORBUG-11501
Change-Id: Id9e261fa8d429fead4a2cd5fd7398aa6e1e8c13d
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>