guix: use autoconf 2.71

This allows for building with newer targets, like arm64-apple-darwin, due to
having a newer bundled config.guess and config.sub.

contrib/guix/manifest.scm

@@ -579,7 +579,7 @@ inspecting signatures in Mach-O binaries.")
         ;; Build tools
         gnu-make
         libtool
-        autoconf
+        autoconf-2.71
         automake
         pkg-config
         bison