make: buildtest with "-Wall -Wextra -Werror"

Make buildtests more pedantic by supplying `-Wall -Wexta -Werror` by default.
This can be disabled through `make W_ALL=0 W_EXTRA=0 W_ERROR=0`, resp.,
or with `make W_NONE=1`, which disables every extra flag.
With `make PEDANTIC=1 PEDANTIC_ERRORS=1` you can make the buildtest even more
pedantic.

Also the generic make variable `W_FLAGS` is added, through which extra
`CFLAGS` can be supplied in the normal `make all`.