Suppress "unknown compiler" warning for MSVC.
Suppress "unknown compiler" warning for MSVC.

Visual Studio doesn't need any special compiler flags
to turn on C++11 support, so we don't need to do
anything special here, but we shouldn't output a
scary "this compiler isn't supported" warning.
Closes salilab/imp#940.

salilab/rmf,salilab/rmf,salilab/rmf,salilab/rmf