fix(ci): use GitHub mirror for MPFR

Author: oscarbenjamin

bin/build_dependencies_unix.sh

@@ -273,7 +273,13 @@ else
   echo --------------------------------------------
   echo
 
-  curl -O https://ftp.gnu.org/gnu/mpfr/mpfr-$MPFRVER.tar.gz
+  if [ $USE_GMP_GITHUB_MIRROR = "yes" ]; then
+    git clone https://github.com/oscarbenjamin/gmp_mirror.git
+    cp gmp_mirror/mpfr-$MPFRVER.tar.xz .
+  else
+    curl -O https://ftp.gnu.org/gnu/mpfr/mpfr-$MPFRVER.tar.gz
+  fi
+
   tar xf mpfr-$MPFRVER.tar.gz
   cd mpfr-$MPFRVER
     ./configure --prefix=$PREFIX\