LU-11946 build: no yaml check during configure --enable-dist

If the yaml libraries are not found, the error is fatal, and prevents
the sources from being packaged.

This check is unnecessary when sources are being packaged, so this patch
disables the test when configure is run with --enable-dist.

Change-Id: I160e0d54efc59480d2f830607467dbc9f34c9de3
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-on: https://review.whamcloud.com/34812
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>