Consider LDFLAGS while linking

[mbargull]

Adds LDFLAGS to the linking step.

A couple of further suggestions/questions:

1. With the current change the link arguments are $(LDFLAGS) -L..
   An alternative could be to have LDFLAGS+=.L and then use $(LDFLAGS) afterwards.
2. There are these conditional LIBS+=-fsanitize=* lines.
   Those got me a little confused; these should be compiler flags and having them in CFLAGS should suffice, shouldn't it?
3. How do you feel about changing CPPFLAGS=-DHAVE_KALLOC to CPPFLAGS+=-DHAVE_KALLOC?
   This way CPPFLAGS from the environment can be reused but the user still has the possibility to override via make CPPFLAGS=....
4. (Less important) How do you feel about changing CFLAGS=... to CFLAGS?=...?
   With that, custom CFLAGS don't have to be explicitly passed on as make arguments.