libglib2: disable compiler warnings

By default, libglib enables some fairly aggressive warnings, treated
as errors. In particular, the -Wformat=2 warning triggers a warning
due to the return value of the ngettext() macro from uClibc libintl
stub not being understood as being potentially a format string.

So, before we enable the stub libintl in uClibc, we disable such
warnings. A bug will be reported to upstream uClibc to get the actual
bug fixed, but disabling compiler warnings treated as errors is anyway
a good thing in the context of Buildroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>