Allow to compile with Visual Studio 2010

For https://bugzilla.gnome.org/show_bug.cgi?id=666491

This patch adds project files to compile and debug libxml2 using Visual
Studio 2010. Only few minor changes have been made to the actual source
code.

This patch also requires for the iconv package to be compiled with visual
studio 2010 which has been submitted to the iconv project (see:
https://savannah.gnu.org/bugs/?35088)