Force wxWidgets to version 3.0.0 or later and OSX build fixes.

* Add version detection to FindwxWidgets.cmake
* Convert commands and macro names to lower case in FindwxWidgets.cmake.
* Use find_package_handle_standard_args() to handle version comparison.
* Update FindPackageHandleStandardArgs.cmake from CMake 3.0.2 to get the
  latest bug fixes.
* Add an option to ignore building every OSX dependency from source and
  use find_package() the way it is supposed to work unless the developer
  specifically requests building everything from source.