net-mgmt/xymon-server: Fix BUILD with LDAP option

• While converting USE_LDAP=yes to USES=ldap this port was missed. Fix this.
• Utilize OPTIONS framework

PR: 269486
Reported by: dinoex
Approved by: portmgr (just-fix-it)
Fixes: 6e1233b Mk/**ldap.mk: Convert USE_LDAP to USES=ldap