Update cmake minimum required version

[Felix Schwitzer]

When I build the library with a recent cmake (3.27), i get the following warning

CMake Deprecation Warning at CMakeLists.txt:2 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

The required version by this projet is very old (3.1) and compatibility with an actual cmake is no longer guaranteed as stated in the cmake documentation.

Thus I would ask to requiring a more recent cmake.

Bumping the required cmake version to 3.15 gives no deprecation- or policy warnings, but having 3.15 at least could help simplify the handling of compiler warning flags as described by CMP0092.

[Ingo Berg]

I have set it to 3.15.