Only run autogen.sh on yarn install if it is available
Only run autogen.sh on yarn install if it is available

When ovirt-engine-nodejs-modules uses this project's
`package.json` and `yarn.lock` to generate it's cache,
the rest of the project is not available.  The `preinstall`
script needs to be sensitive to this situation.

...yarn preinstall will only run autogen.sh if it exists

mareklibra/userportal,oVirt/ovirt-web-ui,mareklibra/userportal,mareklibra/userportal,oVirt/ovirt-web-ui,mareklibra/userportal,oVirt/ovirt-web-ui