CMake: Fix typos.

Forward port of bf019d765d3993ba265c990a6e3a2e53defe112c (Fix spelling
and typos (non-binary), 2012-11-07) from cmake.git.

Change-Id: I0138bd718acb6029cf165b90c5f6a55241810572
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>