add_flag() doesn't add flags to any C++ files in project

[diablodale]

add_flag() in this project's cmake doesn't add flags to C++ files.
I suspect when c++ files were added to the project, the dev didn't update cmake to support them.

Trivial fix now in my fork. Will link to this issue below. Easy cherry pick.

[diablodale]

commit above.

• cmake changes including bumping up one dot release of cmake to 3.3 due to existing code already uses 3.3 features like $<COMPILE_LANGUAGE
• one source file must be updated due to correctly applied flags. More signed/unsigned mismatches. There are more....