Do some reformatting towards the more modern cmake style.

Clean up some code.

Support linking to gcc compiled wxWidgets by setting
`-D__GXX_ABI_VERSION=` appropriately for clang.

Use the new method of calling `FindwxWidgets.cmake`, the old method
causes some problems now.

Set all necessary policies in both cmake files.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>