cmake: build sample urdf models as shared libs on windows too

Change-Id: Ifdd24b9a9507f8cc37d4820fb61d7baacae50564
jac.hh: fix style

Change-Id: I1fb85cc32c5ac3af2de646114cddc977fcd0aa3f
fix compile error C2582 on windows

The const member prevented the compiler to generate the assignment
operator. This in turn made the urdf examples fail compile on Windows.

It seems the assignement operator is needed on Windows but not on the
other platforms, I do not know why.

Change-Id: Iae6fc015b43e3ede59136d968b96d099e8982a2b
cmake: fix METAPODFROMURDF_EXECUTABLE bing overwritten

Change-Id: Ifae8501bb51acd33733d7db44856583a1579a0c2