straightforward; the version is obtained from the header.
No need to put any prefixes anywhere.

For some reason, Sage has version 1.5.2, but header says 1.6.0.
Debian (packsge libmpfi-dev) has 1.5.3, and the header says the same.

CC: @isuruf

Component: build: configure

Author: Dima Pasechnik

Branch/Commit: 047df13

Reviewer: Isuru Fernando

Issue created by migration from https://trac.sagemath.org/ticket/28295