ci: fix warnings and enable --werror for the mingw build

Fix various warnings regarding unused variables, duplicated
branches etc by adjusting the ifdeffery and some missing casts.

gnulib triggers -Wduplicated-branches in one of the copied files,
disable as that just makes updating the code harder.

The warning indicating missing features are made none fatal through
pragmas. They still show but don't abort the build.

https://bugzilla.gnome.org/show_bug.cgi?id=793729

gio/gcredentials.c

@@ -95,7 +95,10 @@ struct _GCredentials
   ucred_t *native;
 #else
   #ifdef __GNUC__
+  #pragma GCC diagnostic push
+  #pragma GCC diagnostic warning "-Wcpp"
   #warning Please add GCredentials support for your OS
+  #pragma GCC diagnostic pop
   #endif
 #endif
 };