Use hyphens in cmake instead of underscores when appropriate

We prefer hyphens ("-") over underscores inside file names. This should be applied to CMakeLists.txt as well.

Closes #3 (closed).

CMakeLists.txt

 cmake_minimum_required(VERSION 3.14)
 project(exple_cmake_build CXX)
 
-option(DOWNLOAD_DEPS "Enable automatic download of dependencies" ON)
+option(DOWNLOAD_DEPS ON)
 
 # download dependency 'explelibx-common' or find it locally
 if (DOWNLOAD_DEPS)