librep: FTBFS with -Werror=implicit-function-declaration

Imported from Debian bug http://bugs.debian.org/1066440:

Source: librep
Version: 0.92.5-3.1
Severity: serious
Justification: FTBFS
Tags: trixie sid ftbfs
User: <email address hidden>
Usertags: ftbfs-20240313 ftbfs-trixie

Hi,

During a rebuild of all packages in sid, your package failed to build
on amd64.

Relevant part (hopefully):
> /usr/bin/ld: /tmp/ccO0t9oT.o: in function `main':
> ././conftest.c:48:(.text.startup+0xb): undefined reference to `mpz_init'
> collect2: error: ld returned 1 exit status

The full build log is available from:
http://qa-logs.debian.net/2024/03/13/librep_0.92.5-3.1_unstable.log

All bugs filed during this archive rebuild are listed at:
https://bugs.debian.org/cgi-bin/pkgreport.cgi?tag=ftbfs-20240313;<email address hidden>
or:
https://udd.debian.org/bugs/?release=na&merged=ign&fnewerval=7&flastmodval=7&fusertag=only&fusertagtag=ftbfs-20240313&<email address hidden>&allbugs=1&cseverity=1&ctags=1&caffected=1#results

A list of current common problems and possible solutions is available at
http://wiki.debian.org/qa.debian.org/FTBFS . You're welcome to contribute!

If you reassign this bug to another package, please mark it as 'affects'-ing
this package. See https://www.debian.org/Bugs/server-control#affects

If you fail to reproduce this, please provide a build log and diff it with mine
so that we can identify if something relevant changed in the meantime.