Use the same condition to exclude kNet when using SDK installation.
Related to commit 867160628cd170f391c63912a2d7638c2ee13bbb
Always use FindDirect3D.cmake module on WIN32 when DirectX is used.
The module is enhanced so it works with both MSVC and MinGW compilers. On MinGW compiler the module does not actually attempt to search for DirectX SDK, instead it simply assumes that the headers and libraries for Direct3D are made available by the MinGW distribution.
For Travis CI - add CI builds for MinGW + URHO3D_OPENGL=0 combination.
These new CI builds will perform the build test on the DirectX code path which is currently not covered in any of our existing CI.
The CI builds should also improve the detection of any breakage caused by build script changes for MinGW compiler.