[CMake] Compiler flags are now set with target_compile_options()
[CMake] Compiler flags are now set with target_compile_options()

- This is the modern version of setting compiler flags

- Renamed COMPILER_FLAGS to RAZ_COMPILER_FLAGS, to be more readable & less error-prone
- Reformatted flags listing so that they're easier to manipulate; removed the surrounding quotes to avoid manual line breaks
    - Removed ';' to ' ' replacement, since target_compile_options() expects flags with a ';' separator
- Removed duplicated warning flags

Razakhel/RaZ,Razakhel/RaZ