remove caching of the LIBNAME in CMake configuration

[van800]

This change lets the generated run configuration in Rider/CLion to updated as soon as the library name is changed, fixes https://youtrack.jetbrains.com/issue/RIDER-136871

Specifically:

1. Rename "EXTENSION-NAME" to "NEW-NAME" and reloading CMake should update the run-configuration

When the name is CACHED, only manual removal of the cmake-build-debug folder would help to regenerate the run configuration.

There is also another problem with the cached name even outside of Rider/CLion.
If you rename the library and call cmake --build /Users/ivan.shakhov/Projects/MyExtension/cmake-build-debug --target all -- -j 14, using the previously used folder cmake-build-debug, it would build the library with the previously cached name.

[Ivorforce]

cc @enetheru (i can't request review unfortunately)

[enetheru]

I approve, Initially I thought it would be good to have the variable discoverable via the help command but its not an option per-se, something that would be set and left, rather than tuned for different circumstances. So changing it to a normal var is a good plan. Any such variables that are not strictly options should also be changed, like the project one.

[dsnopek]

@enetheru approves, and that's enough for me :-)

[Ivorforce]

Thanks!