buildroot

gtest: fix usage on ARM platforms

Ensure that GTest is compiled with -fPIC to allow linking the static
libraries with dynamically linked programs. This is not a requirement
for most architectures but is mandatory for ARM.

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>