Fix for old versions of msvc

I tried to build last version of oiio (master branch) with msvc10 (Windows 7, 64 bits build) and run into pugi build errors.

I found [this commit] (https://github.com/OpenImageIO/oiio/commit/d17fada6838ef9436c4c0b658f38e2fe9f95c13d#diff-510da392f41994333917b2a3d928375d
) that has been reverted by this [more recent commit](https://github.com/OpenImageIO/oiio/commit/844350270ba4b8f151e872cf68bda2a9f79fb2cc).

I don't know if the revert was done on purpose, but re-apply the corrections and modify pugixml.cpp the same way fixe msvc10 build.