Make Security auto-config conditional on spring-security-config

SecurityAutoConfiguration, via its import of
AuthenticationManagerConfiguration, requires spring-security-config
to be on the classpath as AuthenticationManagerConfiguration extends
GlobalAuthenticationConfigurerAdapter from spring-security-config.

This commit makes SecurityAutoConfiguration conditional on the
presence of GlobalAuthenticationConfigurerAdapter so that the
auto-configuration will be disabled in its absence rather than causing
an app to fail to start.

Closes gh-2046

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/security/SecurityAutoConfiguration.java

@@ -27,6 +27,7 @@ import org.springframework.context.annotation.Import;
 import org.springframework.security.authentication.AuthenticationEventPublisher;
 import org.springframework.security.authentication.AuthenticationManager;
 import org.springframework.security.authentication.DefaultAuthenticationEventPublisher;
+import org.springframework.security.config.annotation.authentication.configurers.GlobalAuthenticationConfigurerAdapter;
 import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
 
 /**
@@ -41,9 +42,11 @@ import org.springframework.security.config.annotation.web.configuration.WebSecur
  * <code>@Configuration</code> of type {@link WebSecurityConfigurerAdapter}.
  *
  * @author Dave Syer
+ * @author Andy Wilkinson
  */
 @Configuration
-@ConditionalOnClass(AuthenticationManager.class)
+@ConditionalOnClass({ AuthenticationManager.class,
+		GlobalAuthenticationConfigurerAdapter.class })
 @EnableConfigurationProperties
 @Import({ SpringBootWebSecurityConfiguration.class,
 		AuthenticationManagerConfiguration.class })