If port is setting PKGNAMEPREFIX=${FOOPKGNAMEPREFIX}, but .include's this
file before <bsd.port.options.mk>, options handling gets broken because
options will be loaded from OPTIONSFILE which is based on UNIQUENAME and
thus PKGNAMEPREFIX, which is empty at that point. Prevent this: ensure
that FOOPKGNAMEPREFIX is defined in advance for ports that use it to set
their PKGNAMEPREFIX and having OPTIONS.