mail/postfix: Add option for EAI

The Postfix SMTPUTF8 README [1] makes reference to EAI support being enabled
if ICU is installed on the system, implying (granted, implicitly) that EAI is a
non-compulsory feature. 

Further, there appears no explicitly recommendations, warnings or notices that
EAI should or must not be disabled, is otherwise unsupported in that
configuration, and the README additionally provides instructions on how to
explicitly disable EAI support, consistent with the ability to enable/disable
other non-compulsory features:

  To force Postfix to build without SMTPUTF8, specify:
    $ make makefiles CCARGS="-DNO_EAI ..."

Accordingly:

- Add an OPTION (EAI) to enable/disable the functionality and it's associated
  dependency, leaving it ENABLED by default for package users.

[1] http://www.postfix.org/SMTPUTF8_README.html

Reviewed_by:	ohauer (maintainer)
Approved by:	ohauer (maintainer)
DiffRev:	DXXX