compile.sh: use DependencyMirror repo for downloading gmp

gmplib.org has banned access from all Microsoft IPs, meaning it's not possible to download needed packages in automatic builds, so we have to put them here instead.

compile.sh

@@ -532,7 +532,7 @@ function build_gmp {
 	if cant_use_cache "$gmp_dir"; then
 		rm -rf "$gmp_dir"
 		write_download
-		download_file "https://gmplib.org/download/gmp/gmp-$GMP_VERSION.tar.bz2" "gmp" | tar -jx >> "$DIR/install.log" 2>&1
+		download_file "https://github.com/pmmp/DependencyMirror/releases/download/mirror/gmp-$GMP_VERSION.tar.xz" "gmp" | tar -Jx >> "$DIR/install.log" 2>&1
 		echo -n " checking..."
 		cd "$gmp_dir"
 		RANLIB=$RANLIB ./configure --prefix="$INSTALL_DIR" \