* Build with cmake in appveyor
* Install content files and tests binary to bin
* Install SDL2.dll on windows
* Build RecastDemo as GUI application on Windows and Mac OS
* Copy SDL2.dll to current binary dir
The GCC version used by Travis is old and appears not to support all the
warnings we would like to get, so reenable Clang. Additionally make lots
of cleanups to Travis config and make the build a lot faster too.
Also change both Travis and AppVeyor builds to treat warnings as errors,
and include VS2017 in the matrix for AppVeyor. I have also removed
VS2010 because it is ancient at this point.
Updated Premake to alpha12 and fixed the premake build file
appropriately.
