Update cmake configuration to be able to build both shared and static…

[hobbes1069]

… libraries at the same time.

This has only been tested on Fedora linux and probably needs some tweaking for other platforms.

[jbeder]

Couple high-level questions:

1. Why do you want to build both static and shared libraries at the same time?
2. Can you find a way to test this on other platforms? I don't have the bandwidth to do so, but don't want to accept a patch that doesn't work cross-platform.

[hobbes1069]

The shared library is for all of the packages within the distro to be compiled against, but because yaml-cpp updates frequently break API/ABI, we need something more stable for those developing their own applications. If they use the static library then we don't immediately break their applications with a distro package update.

[hobbes1069]

It should work for any linux distro so perhaps just adding a if(LINUX) conditional requirement to it would be sufficient.