devendoring fmt (if selected)

[dvzrv]

CMakeLists.txt: Introducing an option to build against a system provided fmt version (above 5.1.0).

[jpcima]

It's a good idea. I will just apply over this a set of fixes if you don't mind.

• rename the setting in the prefixed naming scheme: ADLplug_USE_SYSTEM_FMT
• define FMT_USE_USER_DEFINED_LITERALS=0 in both the cases.
• I will drop fmt requirement to 5.0.0