Merge branch 'build_cmake' of https://github.com/TcT2k/wxWidgets

Add CMake-based build system.

Merge the original branch without any changes except for resolving the
conflict due to moving the contents of .travis.yml to a separate file by
propagating the changes done in this file since then to the new script
and rerunning ./build/update-setup-h and ./build/cmake/update_files.py
to update the file lists changed in the meanwhile.

Closes https://github.com/wxWidgets/wxWidgets/pull/330

docs/readme.txt

@@ -36,6 +36,7 @@ include:
 
 Some of the other improvements:
 
+- wxWidgets can now be built with CMake too.
 - Strings can now be translated differently depending on their context.
 - Converting between wxString and UTF-8 encoded std::string is now
   simpler and unsafe wxString can now be disabled on the opt-in basis