This patch is for 1.13 version.

This patch adds a boolean parameter to the multijob phase configuration, with label Trigger only if SCM changes.
When enabled, the plugin launch the job only if it has scm changes.
When disabled, and no SCM changes, the plugin doesn't launch it.

If the parameter/variable hudson.scm.multijob.force.build.always is true, the job is always built.

Additionally, fixed a bug, in MultijobBuild.java:

.append("'/ readonly>") with .append("' readonly/>")

To apply the patch, from the same folder as pom.xml:
patch -p1 < ../jenkins-multijob-plugin-1.13.1-patch

I hope this patch is useful for someone.