BUILD: Makefile: the compiler-specific flags should all be in SPEC_CFLAGS

BUILD: Makefile: the compiler-specific flags should all be in SPEC_CFLAGS

We used to have -Wall -Wextra -Werror in COPTS which are flags fed by
the various USE_* options, and all other warnings in SPEC_CFLAGS. This
makes it impossible to remove these -W* entries (typically -Wextra).

Let's move these 3 flags into SPEC_CFLAGS where they should have been.
Now it's possible to override SPEC_CFLAGS to match any compiler's
specificities, or to clear all warnings at once, or to replace them
all with "-w" to silence warnings.