fontconfig: disable -Werror in scandir test

On Cortex-A15, A12 and A7 -Werror is unusable because by default
the follwing type of warning is issued:

warning: switch -mcpu=cortex-a15 conflicts with -march=armv7-a switch

The proper fix would be to select -march=armv7ve by default for
-mcpu=cortex-a15 and -mcpu=cortex-a7, but that would break any toolchain
which contains gcc prior to 4.9 which is the first version implementing
that -march.

Signed-off-by: Eddy Petrișor <eddy.petrisor@gmail.com>