Various build script fixes

Added BUILD_SHARED_LIBS instead of hardcoding to STATIC or SHARED.

(Probably) fixed FindEEL.cmake, in case someone tries to use it...

cfg-all sets BUILD_SHARED_LIBS to OFF, since MXE doesn't build DLLs.

Eelium is now built and installed as a shared library (static or dynamic),
so host applications can use it along with EEL. (Previously, Eelium was
only available through the 'eel' executive, which was statically linked
with it!)