Update mod2c submodule to latest master

pramodk · pramodk · commit 03740cc6953e · 2019-11-06T14:24:19.000+01:00
diff --git a/external/mod2c b/external/mod2c
@@ -1 +1 @@
-Subproject commit 0a444f413b3b0382c1c6516185b0f029b1f6da7b
+Subproject commit 6f1561f963e1732dbe202112a8837b47b2b98f22
