A cef3 build with gcc5 fails with warnings treated as errors.

bignum.cc, image_util.cc: strict-overflow warning
gtest-typed-test.h: unused-variable warning
SaturatedArithmetic.h: parentheses warning

Compiles for qemux86 with gcc 5.2.0 and still compiles on
armv7a with gcc linaro-4.9.4.
Note that the resulting binary has not been tested on a target.

Signed-off-by: Max Krummenacher <max.oss.09@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>