mpfr: switch to more reliable URL (#4139)
mpfr: switch to more reliable URL (#4139)

* Add list_url to mpfr

* Switch to download directly from GNU

* http -> https

krafczyk/spack,matthiasdiener/spack,LLNL/spack,tmerrick1/spack,krafczyk/spack,EmreAtes/spack,iulian787/spack,TheTimmy/spack,TheTimmy/spack,TheTimmy/spack,lgarren/spack,krafczyk/spack,krafczyk/spack,EmreAtes/spack,iulian787/spack,iulian787/spack,matthiasdiener/spack,EmreAtes/spack,EmreAtes/spack,iulian787/spack,tmerrick1/spack,mfherbst/spack,mfherbst/spack,lgarren/spack,lgarren/spack,skosukhin/spack,LLNL/spack,tmerrick1/spack,EmreAtes/spack,iulian787/spack,TheTimmy/spack,LLNL/spack,tmerrick1/spack,krafczyk/spack,skosukhin/spack,mfherbst/spack,LLNL/spack,lgarren/spack,mfherbst/spack,tmerrick1/spack,TheTimmy/spack,skosukhin/spack,LLNL/spack,matthiasdiener/spack,matthiasdiener/spack,mfherbst/spack,lgarren/spack,skosukhin/spack,matthiasdiener/spack,skosukhin/spack