gl_WARN_ADD does not work with -Wno-xxxx warnings

[Christophe Fergeau]

Hi,

gcc handling of -Wno-xxxx flags is different from its handling of -Wxxxx
flags as described in
http://gcc.gnu.org/onlinedocs/gcc-4.8.1/gcc/Warning-Options.html#Warning-Options
:

"When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC emits a diagnostic stating that the option is not
recognized. However, if the -Wno- form is used, the behavior is slightly
different: no diagnostic is produced for -Wno-unknown-warning unless other
diagnostics are being produced."

This means that with gcc, gl_WARN_ADD([-Wno-unknown-warning]) will always
succeed, leading to issues like
http://www.redhat.com/archives/libvir-list/2012-April/msg00000.html

I guess that when gl_WARN_ADD is used with -Wno-unknown-warning, it should
test as well the corresponding -Wunknown-warning flag, though I don't know
how well this will work with non-gcc compilers.

Christophe

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