problems building emacs with libgpm & gnutls on an old Debian system (Jessie)

[Eric S Fraga <e.fraga@ucl.ac.uk>]

Hello,

Without giving all the gory details, I am trying to build a recent emacs
28.x version (from git) on an old computer running Debian Jessie,
i.e. Linux circa 2014-2015.

I can configure and compile Emacs if I use --with-gnutls=ifavailable but
not having libgnutls-dev or libgmp-dev installed.  This Emacs then runs
but has problems with accessing email (gnus).  I can live with this to
some degree as it meets one of my main requirements which is for writing
prose.  But it would be nice to have email on it as well.

If I install the gnutls library, this brings in libgmp and then Emacs
does not build.  I get error messages along the lines of:

,----
| floatfns.o: In function `Fabs':
| /home/ucecesf/git/emacs/src/floatfns.c:279: undefined reference to `mpz_sgn'
| /home/ucecesf/git/emacs/src/floatfns.c:281: undefined reference to `mpz_neg'
`----

The strange thing is that mpz_sgn, for instance, is a #define in gpm.h
but is not in gmpxx.h.  Both of these include files come from
libgmp-dev.  It could be that Emacs's configure script picks up the
wrong one?

The versions of the two packages in question are:

libgmp-dev:armhf        2:6.0.0+dfsg-6
libgnutls28-dev:armhf   3.3.8-6+deb8u7   

Am I doing something incredibly stupid (I have RTFMed but haven't seen
anything that would help)?  Do I need to build those packages from
source maybe?   

Thank you.
-- 
Eric S Fraga via Emacs 28.0.50 & org 9.3.7 on Debian bullseye/sid