Makefiles fixes

1. Implement global GENERIC_CFLAGS variable to easily change common default CC flags (-std=c99 -Wall -Wextra -pedantic).

2. Implement "fast" target (make fast) to avoid excessive libcodecs traversing (specially for gegel).

3. Make audio_*.{c.h} files depend on each other to trigger rebuild properly.

4. Fix massive builtin.o substitutions in several Makefiles.