Add more warnings.

While these didn't find bugs here, there have been bugs found in
samtools with these options.

.cirrus.yml

@@ -141,7 +141,7 @@ rocky_task:
     LC_ALL: C
     CIRRUS_CLONE_DEPTH: 1
     USE_CONFIG: yes
-    CFLAGS: -std=gnu90 -Wformat -Wformat=2
+    CFLAGS: -std=gnu90 -Wformat -Wformat=2 -Wextra -Wno-sign-compare -Wno-unused-parameter -Wno-missing-field-initializers -Wno-empty-body
 
   # NB: we could consider building a docker image with these
   # preinstalled and specifying that instead, to speed up testing.

test/test-bcf-sr.c

@@ -179,6 +179,7 @@ int main(int argc, char *argv[])
                 break;
             case 'h':
                 usage(EXIT_SUCCESS);
+                // fall-through
             default: usage(EXIT_FAILURE);
         }
     }