libnss: add build workaround for older gcc versions

nss uses some #pragma directives that require newish (>= 4.6) versions
of gcc, so disable Werror when building with older versions to avoid the
build from breaking unfairly.
However make this a gcc >= 4.8 conditional since the kludge is called so
(NSS_NO_GCC48) and other bits might depend upon it.

Upstream bug https://bugzilla.mozilla.org/show_bug.cgi?id=1226179

Fixes:
http://autobuild.buildroot.net/results/ebd/ebd0647eb1eef41a7fb6e5f5bab18d4d12168661/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>