CMakeLists.txt: fix warning with -Wdev
CMake Warning (dev) at CMakeLists.txt:62 (option):
Policy CMP0077 is not set: option() honors normal variables. Run "cmake
--help-policy CMP0077" for policy details. Use the cmake_policy command to
set the policy and suppress this warning.
For compatibility with older versions of CMake, option is clearing the
normal variable 'tiff-install'.
This warning is for project developers. Use -Wno-dev to suppress it.