Only try to auto-configure Jetty when jetty-webapp is on the classpath

Closes gh-4089

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/EmbeddedServletContainerAutoConfiguration.java

@@ -21,6 +21,7 @@ import javax.servlet.Servlet;
 import org.apache.catalina.startup.Tomcat;
 import org.eclipse.jetty.server.Server;
 import org.eclipse.jetty.util.Loader;
+import org.eclipse.jetty.webapp.WebAppContext;
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
@@ -82,7 +83,7 @@ public class EmbeddedServletContainerAutoConfiguration {
 	 * Nested configuration if Jetty is being used.
 	 */
 	@Configuration
-	@ConditionalOnClass({ Servlet.class, Server.class, Loader.class })
+	@ConditionalOnClass({ Servlet.class, Server.class, Loader.class, WebAppContext.class })
 	@ConditionalOnMissingBean(value = EmbeddedServletContainerFactory.class, search = SearchStrategy.CURRENT)
 	public static class EmbeddedJetty {