www/nginx's HTTP_AUTH_KRB5 option is not fully implemented. This patch makes it build with security/krb5 and security/heimdal.
- Enable the Kerberos selection options, which have been commented out ever since they were first committed
- Move these options out of the HTTP_AUTH_KRB5 conditional
- make config now saves the selected option, if any
- the build dependency on the selected Kerberos does now exist in poudriere
- Update the patch for the spnego-http-auth-nginx-module config file to use the current GitHub commit from the Makefile
- Fix the same patch to not assume that -lgssapi is always there
- Force the nginx build system to admit the existence of CPPFLAGS, necessary to build with Heimdal