Consider restructuring the various cflag options

[peterebden]

Right now we have four; dbg/opt and c/c++. The only way to override them is in their entirety which often means you have to repeat two (or all four) of them in order to change one thing (e.g. say you use --std=c++17 globally but you have one package with some third-party code that needs --std=c++03).

I wonder if this should get broken out to different areas, e.g. release/debug specific (-O3 vs. -g3 etc), the language standard, and additional flags (mostly warnings/errors usually). We'd need two of the first (dbg/opt) and of the second (c/c++) and just one of the latter, so it doesn't actually increase the number of options much, but would make it quite a bit more flexible and less repetitive.