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
We need to remove the directory containing sh.exe from the PATH as
otherwise mingw32-make seems to use it and not cmd.exe for executing the
commands in the makefile, but the wrong directory was being removed in
appveyor.yml, having no effect.
Fix this and undo 5f2ccde269 which
compensated for it by (almost) clearing the PATH entirely which, in
turn, broke one of our unit tests (and could break other things in the
future, as having an empty PATH is rather unexpected).
Also use the full path for head program now that its directory is not in
the PATH any more. Alternative could be to use PowerShell, but while
powershell -Command "'gcc','mingw32-make','ar'|%%{ iex \"$_ --version\"|select -first 1 }"
does work, it looks really ugly, so avoid it for now.
See https://github.com/wxWidgets/wxWidgets/pull/142
See #17274.
There is no need to rebuild everything if only the documentation files
have changed.
Unfortunately there doesn't seem to be any way to do the same thing for
Travis CI yet, see https://github.com/travis-ci/travis-ci/issues/6301
Unlike Travis CI, Appveyor doesn't do this automatically, but we will
need them soon (the currently existing Catch submodule is not used by
Appveyor builds but, arguably, should be too).
