Only adding the c++11 flag when the extlib is written in c++.
Only adding the c++11 flag when the extlib is written in c++.

This avoid a lot of warnings when building metis.

Signed-off-by: Étienne Schmitt <etienne.schmitt@inria.fr>

Former-commit-id: 85c5e78cedf4ac19fa655567c8a4ddb130c96f26
hdeling/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,Anatoscope/sofa,Anatoscope/sofa