MSVC: Static gflags library doesn't work

```
Benchmark.h
DECLARE_bool(benchmark);

Benchmark.cpp
DEFINE_bool(benchmark, false, "Run benchmarks.");

Benchmark.cpp(35): error C2370: 'gflags::FLAGS_benchmark': redefinition; different storage class
Benchmark.h(34): note: see declaration of 'gflags::FLAGS_benchmark'
```
Because `DEFINE_VARIABLE` always add `GFLAGS_DLL_DEFINE_FLAG`