Declare specific version for CMake

[emanuelb]

Building the android app show many warnings such as:

/home/appuser/app/eclair/eclair-mobile/app/CMakeLists.txt : C/C++ release|armeabi-v7a : CMake Warning at /home/appuser/app/sdk/ndk/23.0.7196353/build/cmake/android-legacy.toolchain.cmake:427 (message):
  An old version of CMake is being used that cannot automatically detect
  compiler attributes.  Compiler identification is being bypassed.  Some
  values may be wrong or missing.  Update to CMake 3.19 or newer to use
  CMake's built-in compiler identification.
Call Stack (most recent call first):
  /home/appuser/app/sdk/ndk/23.0.7196353/build/cmake/android.toolchain.cmake:55 (include)
  /home/appuser/app/sdk/cmake/3.10.2.4988404/share/cmake-3.10/Modules/CMakeDetermineSystem.cmake:94 (include)
  CMakeLists.txt

By declaring a specific version for cmake, see:
https://developer.android.com/studio/projects/install-ndk#vanilla_cmake
a newer version can be declared, also using static value will help to reproducible-builds as well #232 (as the same version will be used as declared in build.gradle file)