Fix custom module paths

[williamposey]

The custom module path variables in CMakeLists.txt should be directly referenced, rather than their values. For example, the value of KEY_CUSTOM_PATH is checked here rather than the variable itself.

The custom module path variables in src/CMakeLists.txt should be referenced by value with their inclusion in aux_source_directory() rather than referenced directly. For example, the KEY_CUSTOM_PATH variable is refenced here directly when it should have its value referenced using expansion (${KEY_CUSTOM_PATH})