We didn't document using wxwin.m4 at all anywhere, so do it at least
minimally now (we still need a separate page documenting its full
functionality) and also show a simple makefile using wx-config.
Closes https://github.com/wxWidgets/wxWidgets/pull/2338
Briefly explain how to check that wxWidgets is installed correctly.
This is again redundant with the information already present elsewhere,
but it seems worth repeating it here.
Currently this simply links to the existing port-specific installation
instructions, but this should provide a less confusing and more
prominent entry point than the existing topics.
