Added LIB_DEST_DIR option to CMakeLists.txt

I needed to patch CMakeLists.txt so that I could [package](https://github.com/TravisPaul/spec-wjelement) WJElement for Fedora which expects shared libraries to be installed to /usr/lib64 on 64bit machines. I thought this option might be useful for others as well.

WJElement is great! I am making heavy use of the schema validation. Thanks for open-sourcing the library!