Consistent CMake variable conventions

[ekilmer]

CMake doesn't exactly have an official convention for variable names or capitalization outside of what the docs say you will get, but there are auto-generated variable names that come from the same capitalization as used in the project(<name> ...) declaration, like _SOURCE_DIR, CMAKE_DISABLE_FIND_PACKAGE_, and others.

While the project is still young, I think we should refactor all cases of variables to use the same capitalization as used in the project(sleigh ...) declaration.