Macro to provide an option dependent on other options.
This macro presents an option to the user only if a set of other conditions are true. When the option is not presented a default value is used, but any value set by the user is preserved for when the option is presented again. Example invocation:
CMAKE_DEPENDENT_OPTION(USE_FOO "Use Foo" ON "USE_BAR;NOT USE_ZOT" OFF)
If USE_BAR is true and USE_ZOT is false, this provides an option called USE_FOO that defaults to ON. Otherwise, it sets USE_FOO to OFF. If the status of USE_BAR or USE_ZOT ever changes, any value for the USE_FOO option is saved so that when the option is re-enabled it retains its old value.