SDL incorrectly detects X11_XRANDR with CMake, does not compile on Solaris 10

[joncox123]

By default, SDL2 tries to enable support for XRANDR with the SDL_X11_XRANDR option in CMake, for example. The problem is that Solaris 10 does not actually support XRANDR in practice, even though CMake and configure are detecting it. As a result, compilation will fail during the build process.

Therefore, if the Solaris or SunOS platform is detected in sdlplatform.cmake, I suggest a warning message is written and SDL_X11_XRANDR is set to "OFF" by default. I am not a build system engineer, so if you have a more elegant way of handling this, I am all ears.