Cmake install improvements

Slight tweaks to the cmake files for two major purposes:
- Having a working `install` on win32 plaftorm;
- Providing a YajlConfig.cmake for easy use of the package through cmake (see [the documentation](http://www.cmake.org/Wiki/CMake/Tutorials/How_to_create_a_ProjectConfig.cmake_file) for further information).

I've tested it on windows 7 32 bits with Visual Studio, the changes are only impacting the installed package.