Modernize CMake script

[benvenutti]

Hello there, in this PR I did a small refactoring on the original CMake script to give it a bit of a modern CMake approach. To achieve that, the minimum version of CMake was bumped to 3. The idea was to:

• remove compiler specific flags;
• avoid global project modifiers (for example, include_directories);
• avoid globs (those can be a pain when working with build caches).

I also added a gitignore file (to help on out-of-source builds) and some extra information of the README file.