Fix minor issue in the CMake config to make error message for users more useful.
ENDRELEASENOTES
Previously, if the c++ standard was not 17 or 20 it was not detected, and we failed hard with ROOT C++ could not be detected, which doesn't tell the user anything. Demoting that to a warning makes the actually useful message that we have appear.
BEGINRELEASENOTES
ENDRELEASENOTES
Previously, if the c++ standard was not 17 or 20 it was not detected, and we failed hard with
ROOT C++ could not be detected, which doesn't tell the user anything. Demoting that to a warning makes the actually useful message that we have appear.