cmake dependency refactoring / manage own dependencies / WIP

**This will significantly change the build system**

This is a first step to bring in proper multiplatform support, managed solely from CMake. To enable this, we will manage all library dependencies ourself, instead of relying on package managers like brew or apt-get.

I.e.: instead of installing dependencies via brew, *use CMake in the extdep folder* (see the Readme there)

In the first step, only libraries CryptoPP and Json-RPC-CPP are included; it is a test to see that this workflow will work for everyone. This will also fix #453

The included libraries work fine under Ubuntu and OS X. Windows support is not there yet, however the patches from @debris in this PR are necessary to enable Windows support later (see #469)



What is still missing:
* Most libraries still need to be CMake-ified, so brew/apt-get  is still needed for them. 
* An automatic compile step will be added to provide pre-compiled libraries from buildbot, so that developers won't have to compile libraries themselves
* buildbot will likely complain about this PR, let's see.


move to condition based on feature flag rather than existence of dependency