Clean up FindUrho3D.cmake module.
Add new build options to override 64-bit Linux package setup for Debian and Redhad -based host systems.
For Travis CI - fix 64-bit RPM package generation to install library in /usr/lib64, also fix external project build test using Urho3D library from SDK.
Refactor all CMake toolchains to reset CMAKE_INSTALL_PREFIX variable.
Add new build options for specifying tool-prefix and target-sysroot in cross-compiling build.
Add new IOS_SYSROOT variable to store the path to iOS sysroot and use it to set CMAKE_FIND_ROOT_PATH variable.
Now also generate pkg-config Urho3D.pc for iOS platform.
For Travis CI - simplify SDK installation logic. Use DESTDIR=~ to avoid sudo.
For Travis CI - switch CI build to use OSX build environment.