We're setting these using SET() in extlib/CMakeLists.txt, so having them
as actual options causes CMake to print a bunch of warnings like this:
CMake Warning (dev) at extlib/zlib-ng/CMakeLists.txt:133 (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 'WITH_SSE2'.
This warning is for project developers. Use -Wno-dev to suppress it.
[extlib] CMakeLists.txt: SET(WITH_VPCLMULQDQ ON)