LU-16502 lutf: add proper config option and fix bugs

LU-16502 lutf: add proper config option and fix bugs

LUTF did not have a proper configuration option. Since
no message was printed at configure time, this made it
hard to debug why LUTF was not being built.

Fix a few minor bugs in headers that prevented shared
libraries from being `import`ed by python.

Fix a small Clang error in liblutf_agent.c.

Test-Parameters: @lnet
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I6680b203bef08b7afa326a1cbe30c96b5c29e95c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53200
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>