mir fails to build with GCC 5

as seen in silo16:

https://launchpadlibrarian.net/212029958/buildlog_ubuntu-wily-amd64.mir_0.14.0%2B15.10.20150715-0ubuntu2~gcc5.1_BUILDING.txt.gz

make[3]: Entering directory '/«BUILDDIR»/mir-0.14.0+15.10.20150715/obj-x86_64-linux-gnu'
/usr/bin/cmake -E cmake_progress_report /«BUILDDIR»/mir-0.14.0+15.10.20150715/obj-x86_64-linux-gnu/CMakeFiles
[ 10%] Building CXX object src/protobuf/CMakeFiles/mirprotobuf.dir/google_protobuf_guard.cpp.o
cd /«BUILDDIR»/mir-0.14.0+15.10.20150715/obj-x86_64-linux-gnu/src/protobuf && /usr/bin/x86_64-linux-gnu-g++ -DLOG_NDEBUG=1 -DMESA_EGL_NO_X11_HEADERS -DMIR_VERSION_MAJOR=0 -DMIR_VERSION_MICRO=0 -DMIR_VERSION_MINOR=14 -Dmirprotobuf_EXPORTS -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -pthread -g -std=c++14 -Werror -Wall -fno-strict-aliasing -pedantic -Wnon-virtual-dtor -Wextra -fPIC -fPIC -I/«BUILDDIR»/mir-0.14.0+15.10.20150715/include/common -I/«BUILDDIR»/mir-0.14.0+15.10.20150715/src/include/common -I/«BUILDDIR»/mir-0.14.0+15.10.20150715/obj-x86_64-linux-gnu/src/protobuf -o CMakeFiles/mirprotobuf.dir/google_protobuf_guard.cpp.o -c /«BUILDDIR»/mir-0.14.0+15.10.20150715/src/protobuf/google_protobuf_guard.cpp
In file included from /«BUILDDIR»/mir-0.14.0+15.10.20150715/src/protobuf/google_protobuf_guard.cpp:19:0:
/«BUILDDIR»/mir-0.14.0+15.10.20150715/src/include/common/mir/protobuf/google_protobuf_guard.h:37:6: error: '{anonymous}::force_google_protobuf_init' defined but not used [-Werror=unused-variable]
 bool force_google_protobuf_init{(mir::protobuf::google_protobuf_guard(), true)};
      ^
/usr/bin/cmake -E cmake_progress_report /«BUILDDIR»/mir-0.14.0+15.10.20150715/obj-x86_64-linux-gnu/CMakeFiles
cc1plus: all warnings being treated as errors
make[3]: *** [src/protobuf/CMakeFiles/mirprotobuf.dir/google_protobuf_guard.cpp.o] Error 1
[ 10%] src/protobuf/CMakeFiles/mirprotobuf.dir/build.make:75: recipe for target 'src/protobuf/CMakeFiles/mirprotobuf.dir/google_protobuf_guard.cpp.o' failed
make[3]: Leaving directory '/«BUILDDIR»/mir-0.14.0+15.10.20150715/obj-x86_64-linux-gnu'
CMakeFiles/Makefile2:2622: recipe for target 'src/protobuf/CMakeFiles/mirprotobuf.dir/all' failed
make[2]: *** [src/protobuf/CMakeFiles/mirprotobuf.dir/all] Error 2