fixed up new C code to be (mostly) C89-compliant

*mostly*, because the "inline" keyword isn't C standard until C99, but
I'm leaving it in because most compilers support it and it's really
handy. If it becomes an issue, we can deal with it later.

src/main.c

@@ -31,5 +31,6 @@ PyMODINIT_FUNC
 initc_overviewer(void)
 {
     (void)Py_InitModule("c_overviewer", COverviewerMethods);
-    import_array();     // for numpy
+    /* for numpy */
+    import_array();
 }