Introduced global swith BUILD_SHARED_LIBS

Introduced new global build switch BUILD_SHARED_LIBS which defaults to ON. Observe that the special casing of MSVC is removed, i.e. you must invoke cmake as:

    cmake <path> -DBUILD_SHARED_LIBS=OFF

To get a static build, *irrespective of the environment*. Observe that both the full ERT application, and also the Python wrappers require a shared build - i.e. the configure step will fail hard if you combine -DBUILD_ERT or -DBUILD_PYTHON with static libraries.