Revert "Use --sysroot when cross compiling."

This reverts commit de4da665d3205afa239749c41513a315c3831f51.

This feature is not yet ready for release. It needs to be
merged with the CMAKE_OSX_SYSROOT feature.
Merge topic 'revert-CMAKE_SYSROOT' into next

0416c94 Revert "Use --sysroot when cross compiling."