Generic-ify Makefile a bit
* Pull out common arguments across platforms.
* Allow command line overrides for most common variables.
* Allow CROSS and LDFLAGS to be set, to attempt(?) cross-compiling.
* Make existing hardcoded wx-config paths defaults only if no path is set, fail over to `which wx-config` if they don't exist.
Makefile: Make -O2 optional for easier debugging
Add compatibility for wx2.8 Unicode builds

Code will still work with ANSI builds, but Unicode-enabled is
how most distros Linux ship wx I think (I'm not a wx expert though).

I think this all becomes unnecessary again in wx3.0, where everything
is unicode and these upconversions mostly become implicit, but at
present 2.x is still pretty widely distributed.