Setting the variables CMAKE_AUTOGEN_ORIGIN_DEPENDS,
CMAKE_AUTOMOC_COMPILER_PREDEFINES, and CMAKE_AUTOMOC_MACRO_NAMES on the
cmake command line did not work as expected. CMakeGenericSystem.cmake
initialized these variables even if they were defined by the user.

This led to the confusing situation where the cache variant of a
variable had one value (defined on the command line) and the non-cache
variant of a variable had a different value (defined in
CMakeGenericSystem.cmake).

Fix this by checking whether the variables are defined before setting
their default values.

Fixes: #24069