math/units: Add a default option READLINE to tuggle readline support

This makes sure the readline support is tuggled in the build time and not
affected by the libreadline installed or not during build/run time.

PR: 248631
Submitted by: ports-units@shalott.net
Approved by: jharris@widomaker.com (maintainer)
MFH: 2020Q3 (blanket: build, run, dependency fix)