Bug#964848: mpir deprecated in favour of gmp in sagemath

[Dima Pasechnik]

Since at least SageMath 9.1, a default source installation will first
look for GMP
available on the system, and use it if it was found.
Or, in fact, any GMP-compatible package available on the system, such
as MPIR - but the latter is rather unusual; compatibility
is only lightly tested, in the usual autoconf way (i.e. for presense
of headers and library, and the version of GMP to be at least 6.1.0,
by testing presense of certain GMP function))


If none of these was found, MPIR would be built and used, but this
will change in SageMath 9.3, so that GMP would be built,not MPIR, cf
https://trac.sagemath.org/ticket/30752