In the cmake command-line tool, the message() command with no
message mode argument prints the message stderr using the C++ cerr
stream. Since !6105 (merged) and an update by !6369 (merged), we print the newline at the end of
the message using just \n. We've now observed some cases of output
on stdout and stderr getting jumbled when the two go to the same file
descriptor. Previously the newline was printed with endl, which
implicitly flushes. Flush explicitly to restore that behavior.
Fixes: #23155 (closed)
Backport: release