CMake: override option with toolchain variable

[tobtoht]

In a Guix build on master:

CMake Warning (dev) at CMakeLists.txt:406 (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 'BUILD_TESTS'.

CMake Warning (dev) at CMakeLists.txt:464 (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 'STATIC'.

From cmake --help-policy CMP0077:

* The ``OLD`` behavior for this policy is to proceed even when a normal
  variable of the same name exists.  If the cache entry does not already
  exist and have a type then it is created and/or given a type and the
  normal variable is removed.

* The ``NEW`` behavior for this policy is to do nothing when a normal
  variable of the same name exists.  The normal variable is not removed.
  The cache entry is not created or updated and is ignored if it exists.

The expected behavior is that the variables defined in toolchain.cmake.in (such as STATIC) override the default options.