Mark installation of .pdb files as optional

As per this CMake issue http://public.kitware.com/Bug/view.php?id=14600 and related issues/commits, .pdb file generation by VisualStudio projects seems to have changed since CMake 2.8.12. In particular, <LibraryName>.pdb files have disappeared for static libraries, which causes installation to fail because we used to install those files. As a quick fix, I suggest to mark installation of such files as OPTIONAL; anyway, new CMake (and VS) policies for PDB files should be studied in greater detail to find appropriate solution (hoping they are "final" by now ^^).