Prevent C++ + locale + uClibc 0.9.31 + gcc 4.2 to be selected

The problem fixed by 60f945e4 is in
fact not limited to the AVR32 architecture, as reported by Will Newton
on the list. The issue is the combination uClibc 0.9.31 with gcc 4.2,
C++ support and locales.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>