BuildSystem: Really do not pass unknown-warning options to the compiler.

When an older GCC version is called with a too new warning option, GCC exited
with an error and Asterisk was not built. Therefore, the configure script tests
the installed compiler whether it supports that warning option. If not, Asterisk
does not pass it to the installed compiler. However, some compilers (like clang)
do not exit (error) but give just a warning in such a case. Because the compiler
did not exit, Asterisk passed the unknown-warning option.

ASTERISK-27560

Change-Id: Ia9b7747f649b27ff5e9f75c3db3fee4fe7a29621