Hide internal and advanced CMake variables from casual cmake-gui user.
Also in the commit:
- Slightly speed up the configuration step by taking advantage of CMake cache.
- Does not expose URHO3D_64BIT as build option on Android and Raspberry Pi platforms because this variable should be preset based on the chosen compiler toolchain.
- Only provide URHO3D_MKLINK build option when building on a Windows host system and not when cross-compiling using MinGW toolchain.
- Refactor Android and Raspberry Pi CMake toolchains to make them consistent with each other and more user friendly.
Fix MinGW CMake toolchain to also work with Debian-based host system.
For Travis CI - switch CI build to use OS X build environment.