Allow usage of system libraries

Hi,
I've made simple adjustments to cmake build system files to allow user to use system boost and gmock/gtest libraries. Default setting is to use the bundled ones, of course. I think this is harmless for you, but it's quite handy for us, Gentoo maintainers. So, I'll be glad if you merge these patches.
Thanks.
2 space indent. Please fix this everywhere.