supppress unused variable warning for gcc (#372)
supppress unused variable warning for gcc (#372)

gcc complains with: 
```
'stacktraces_enabler' defined but not used [-Wunused-variable] 
static int stacktraces_enabler = []() { 
```
abseil/abseil-cpp,abseil/abseil-cpp,abseil/abseil-cpp,firebase/abseil-cpp,firebase/abseil-cpp,firebase/abseil-cpp,abseil/abseil-cpp,firebase/abseil-cpp