graphics/s2: fix build on big-endian

/usr/bin/c++ -DS2_USE_GFLAGS -DS2_USE_GLOG -Ds2_EXPORTS -I/wrkdirs/usr/ports/graphics/s2/work/s2geometry-0.10.0/src -O2 -pipe -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -mcpu=power7 -isystem /usr/local/include -std=c++11 -O2 -pipe -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -mcpu=power7 -isystem /usr/local/include -std=c++11 -fPIC -Wno-attributes -Wno-deprecated-declarations -pthread -std=c++11 -MD -MT CMakeFiles/s2.dir/src/s2/util/math/exactfloat/exactfloat.cc.o -MF CMakeFiles/s2.dir/src/s2/util/math/exactfloat/exactfloat.cc.o.d -o CMakeFiles/s2.dir/src/s2/util/math/exactfloat/exactfloat.cc.o -c /wrkdirs/usr/ports/graphics/s2/work/s2geometry-0.10.0/src/s2/util/math/exactfloat/exactfloat.cc
/wrkdirs/usr/ports/graphics/s2/work/s2geometry-0.10.0/src/s2/util/math/exactfloat/exactfloat.cc:144:20: error: expected value in expression