Enable Cross-Compilation for iOS

[Treata11]

Greetings,

Cross-compilation for a macOS target was already supported within this framework. To enable iOS compilations, I've introduced a new flag, EMBREE_BUILD_APPLE_FRAMEWORKS, which is set to FALSE by default.

This implementation relies on the iOS-CMake toolchain for building.
My changes should be sufficient; however, the builds fail at the FindTBB.cmake script:

-- Looking for TBB components tbb; ()
CMake Error at common/cmake/FindTBB.cmake:77 (message):
  Unable to find root directory TBB_ROOT-NOTFOUND
Call Stack (most recent call first):
  common/cmake/FindTBB.cmake:462 (rk_tbb_error)
  common/tasking/CMakeLists.txt:33 (find_package)

It's worth mentioning that oneTBB already supports iOS builds with the same approach using iOS-CMake toolchain.