mpfr: switch to more reliable URL (#4139)

* Add list_url to mpfr

* Switch to download directly from GNU

* http -> https

var/spack/repos/builtin/packages/mpfr/package.py

@@ -30,14 +30,15 @@ class Mpfr(AutotoolsPackage):
        floating-point computations with correct rounding."""
 
     homepage = "http://www.mpfr.org"
-    url      = "https://gforge.inria.fr/frs/download.php/latestfile/159/mpfr-3.1.2.tar.bz2"
+    url      = "https://ftp.gnu.org/gnu/mpfr/mpfr-3.1.5.tar.bz2"
 
     version('3.1.5', 'b1d23a55588e3b2a13e3be66bc69fd8d')
     version('3.1.4', 'b8a2f6b0e68bef46e53da2ac439e1cf4')
     version('3.1.3', '5fdfa3cfa5c86514ee4a241a1affa138')
     version('3.1.2', 'ee2c3ac63bf0c2359bf08fc3ee094c19')
 
-    depends_on('gmp@4.1.0:')  # mpir is a drop-in replacement for this
+    # mpir is a drop-in replacement for gmp
+    depends_on('gmp@4.1.0:')  # 4.2.3 or higher is recommended
 
     patch('vasprintf.patch', when='@3.1.5')
     patch('strtofr.patch',   when='@3.1.5')