LU-17836 build: allow builds without libpthread

LU-17836 build: allow builds without libpthread

Configure currently allows for --disable-libpthread it is not
frequently used but may be needed for some users.

Test-Parameters: trivial
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I32049bab8e0f278b4c80fe37839c8c90c45d4c74
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55062
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>