Makefile: clean up CFLAGS and LDFLAGS #254

The way the Makefile treads CFLAGS and LDFLAGS is confusing.

Currently DEFAULT_CFLAGS and DEFAULT_LDFLAGS apply to both debug and release builds, but CFLAGS and LDFLAGS apply only to release builds. Also the DEFAULT_ flavor may be prepended-to via environment variables, but the non-default ones cannot.

Let's try to simplify it and make it more comprehensible.

One idea:

• remove the DEFAULT_ flavored ones
• CFLAGS and LDFLAGS can be initialized from the environment
• the difference between debug and release builds appears only in the relevant make targets

This could also be a place to simplify the sanitizer settings, e.g. by having a single SANITIZE environment variable, e.g. SANITIZE=memory which gets rendered via make to both CFLAGS and LDFLAGS -fsanitize=memory etc.