Add debug & release build options and information message

[RaymiiOrg]

These are two small things I like in my cmakelists:

• one to have specific Debug and Release builds, with specific optimalisations (O2/O0) and the ability for specific flags/options per build type
• a message showing the current compile flags.

I'm unsure how clang handles the -ggdb flag since I've not yet worked with clang, only GCC.

[lenticularis39]

I'm unsure how clang handles the -ggdb flag since I've not yet worked with clang, only GCC.

Clang should support -ggdb according to its documentation, so it should be fine.

[lenticularis39]

Note: CMake sets -O3 -DNDEBUG by itself when building with the Release build type, but these don't show up in CMAKE_lang_FLAGS (and when specified there manually they aren't duplicated, so that's fine).