example fun example shows benefit of header only library

Former-commit-id: fec8188906a36a551cfc1c01d157cf1205c4100b

examples/example_fun/example.cpp

@@ -8,5 +8,9 @@ int main(int argc, char * argv[])
   example_fun(d);
   int i = 4;
   example_fun(i);
+#ifdef IGL_HEADER_ONLY
+  const char * s = "string";
+  example_fun(s);
+#endif
   return 0;
 }