It is common to specify a CMAKE_TOOLCHAIN_FILE and get a warning
for using it despite it not being used.
The WarnUnusedCliUnused test relies on the warning being emitted
each time cmake is run on an existing build. That behavior is changed
by this patch to warn only on the first invokation of CMake, and not
on subsequent invokations (because the variable is in the cache with
the same value). For that test, a clean target is added which clears
the cache and cause the warning to be emitted each time.
As the Ninja generator does not support the feature needed to test
this, it is not tested with that generator.
