Improve cmake build so that external deps are not required

This patch extends cmake such that external-dependencies are no longer required as the system installed packages are found. Tested on Debian 7 32 bit and Raspberry PI ARM. If the FBX sdk is installed, the encoded will be built too. There is still work to do - cmake should be able to generate visual studio and xcode projects ... but I don't think it will at the moment (but it did not before).
I don't expect it to be accepted but would appreciate some comments on what needs doing to make it acceptable as the current cmake system (which requires pre-built external-dependencies does not really work for embedded linux systems). I am far from an expert on cmake, so there are likely better ways of doing things than in this patch.