guile: disable parallel building for reproducibility

pkgs/development/interpreters/guile/default.nix

@@ -38,7 +38,13 @@
     libtool libunistring
   ];
 
-  enableParallelBuilding = true;
+  # According to Bernhard M. Wiedemann <bwiedemann suse de> on
+  # #reproducible-builds on irc.oftc.net, (2020-01-29): they had to
+  # build Guile without parallel builds to make it reproducible.
+  #
+  # re: https://issues.guix.gnu.org/issue/20272
+  # re: https://build.opensuse.org/request/show/732638
+  enableParallelBuilding = false;
 
   patches = [
     ./eai_system.patch
@@ -109,4 +115,3 @@
     '';
   };
 })
-