Fix libunwind symbols missing

[bmyerz]

GLOG (google logging) depends on libunwind. Sometimes libunwind symbols were not found by the linker.

Without the fix

[ 95%] Linking CXX executable hello_world.exe
../../third-party/lib/libglog.a(libglog_la-utilities.o): In function `google::GetStackTrace(void**, int, int)':
utilities.cc:(.text+0x7a6): undefined reference to `_Ux86_64_getcontext'
utilities.cc:(.text+0x7bf): undefined reference to `_ULx86_64_init_local'
utilities.cc:(.text+0x818): undefined reference to `_ULx86_64_get_reg'
utilities.cc:(.text+0x877): undefined reference to `_ULx86_64_step'
collect2: error: ld returned 1 exit status
make[3]: *** [applications/demos/hello_world.exe] Error 1
make[2]: *** [applications/demos/CMakeFiles/demo-hello_world.dir/all] Error 2
make[1]: *** [applications/demos/CMakeFiles/demo-hello_world.dir/rule] Error 2
make: *** [demo-hello_world] Error 2

Proposed fix (c9bc0dd) adds the linking flag explicitly.

[bmyerz]

from travis-ci build

Probably don't make it required

[bmyerz]

Leaving a more explicit note here: when we enable frame pointers when libunwind is not found, we also need to enable them in our build of gperftools

[nelsonje]

I think the right thing to do here is redistribute the correct version of libunwind, perhaps with c++11 modifications. But I've had trouble getting libunwind+gperftools to work with Grappa in recent months, and I'm not going to have more time to debug it. You want to take a look sometime? :-)