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