The new default values WITH_URING:BOOL=OFF, WITH_PMEM:BOOL=OFF imply
that the dependencies are optional.
An explicit request WITH_URING=ON or WITH_PMEM=ON will cause the
build to fail if the requested dependencies are not available.

Last, to prevent a feature to be built in even though the built-time
dependencies are available, the following can be used:

cmake -DCMAKE_DISABLE_FIND_PACKAGE_URING=1
cmake -DCMAKE_DISABLE_FIND_PACKAGE_PMEM=1

This cleanup was suggested by Vladislav Vaintroub.