CMake refactoring

[volkm]

• Consistent CMake argument CMAKE_USE_XY
• Sorted dependencies alphanumerically in storm-config.h and CMakeLists
• Use MATHSAT instead of MSAT, added FindMathSat script
• Use STORM_USE_XERCES instead of STORM_XML_SUPPORT

Could have some conflicts with #602

[sjunges]

Generally, I have included most changes in the refactored CMAKE in the other PR

[volkm]

So we should first merge your PR and then see what changes we could still integrate from this PR?

[sjunges]

Indeed, I would close it for now for clarity; I think I have covered it (except maybe some order of definitions.. but I am cleaning up things either way)