Review Makefile thoroughly, cleanup/docs

[ped7g]

For example there's ifndef KEEP_SYMBOLS while this should maybe also evaluate KEEP_SYMBOLS and check also value?

review all optional variables and their comments (f.e. bundled lua could describe both options)

re-check all the described build flavors (asan, ubsan. ...), maybe add more docs about how-to (you already forgot how to do them, so you will have to figure it out anyway), check other txt notes for valgrind/profiling and move it to makefile docs so it's all in one place.

c++17 vs gnu++17 - maybe detect mingw platform?

Check also how feasible it would be to use just one makefile (also for windows)