DOC: Improve Apple Clang error to include C++14 requirement
DOC: Improve Apple Clang error to include C++14 requirement

This error could be confusing if a sufficiently new Clang version is
used but C++11 or C++03 is used.

thewtex/ITK,hjmjohnson/ITK,richardbeare/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,BRAINSia/ITK,vfonov/ITK,thewtex/ITK,hjmjohnson/ITK,richardbeare/ITK,hjmjohnson/ITK,Kitware/ITK,richardbeare/ITK,thewtex/ITK,richardbeare/ITK,BRAINSia/ITK,Kitware/ITK,BRAINSia/ITK,Kitware/ITK,thewtex/ITK,vfonov/ITK,BRAINSia/ITK,vfonov/ITK,vfonov/ITK,thewtex/ITK,richardbeare/ITK,thewtex/ITK,thewtex/ITK,BRAINSia/ITK,vfonov/ITK,Kitware/ITK,hjmjohnson/ITK,hjmjohnson/ITK,Kitware/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,richardbeare/ITK,Kitware/ITK,BRAINSia/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,Kitware/ITK,richardbeare/ITK