CPT: Add 'compiler' argument.
CPT: Add 'compiler' argument.

Explicitly passing the compiler from Travis or Appveyor allows it to be printed in the log more apparently.
Can further be extended to mark dependencies when building packages, or even copy C++ headers into install.

Signed-off-by: Vassil Vassilev <0b291c0bdb4e22c8b7705ceedf8e168ff73f20e8@cern.ch>

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