Fixes the following warning:
```
CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.
```
See also unittest-cpp/unittest-cpp#192.

Closes: https://bugs.gentoo.org/964647
Signed-off-by: Louis Sautier <sbraz@gentoo.org>