BUILD: makefile: validate support for extra warnings by batches

The makefile takes quite some time to check supported warning options
and that's getting quite annoying. Most of the time all the tested ones
are quite legacy and well supported, so let's first try to validate
them all at once, and only if they fail, test them individually.

Doing so reduces the number of calls to the compiler to ~4 during the
startup, which is much better.
1 file changed