CMake tweaks:

- Include GNUInstallDirs which sets up standard install locations including
  lib64 for mulilib systems.
- Make BUILD_SHARED_LIBS an option instead of a variable which is better for
  use in either the cmake-gui or ccmake gui interfaces.
- Setup a destination for WIN32 runtime DLL's which is also helpful for MinGW
  installs.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1012 99668b35-9821-0410-8761-19e4c4f06640