Fix gflags_compat.h (#11346)

Summary:
Was getting compilation failure with old verison of gflags, examples in https://github.com/facebook/rocksdb/issues/11344.  Perhaps this is new since enabling C++17. Getting rid of std::reference_wrapper from https://github.com/facebook/rocksdb/issues/10729 seems to fix it.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11346

Test Plan: manual, CI

Reviewed By: guowentian

Differential Revision: D44632776

Pulled By: pdillinger

fbshipit-source-id: 5c1f3f79a055698574538b6342c912a627b6d061

util/gflags_compat.h

@@ -15,16 +15,15 @@
 #endif
 
 #ifndef DEFINE_uint32
-// DEFINE_uint32 does not appear in older versions of gflags. This should be
+// DEFINE_uint32 / DECLARE_uint32 do not appear in older versions of gflags.
-// a sane definition for those versions.
+// These should be sane definitions for those versions.
 #include <cstdint>
 #define DEFINE_uint32(name, val, txt) \
   namespace gflags_compat {           \
   DEFINE_int32(name, val, txt);       \
   }                                   \
-  std::reference_wrapper<uint32_t> FLAGS_##name = \
+  uint32_t &FLAGS_##name =            \
-      std::ref(*reinterpret_cast<uint32_t *>(&gflags_compat::FLAGS_##name));
+      *reinterpret_cast<uint32_t *>(&gflags_compat::FLAGS_##name);
 
-#define DECLARE_uint32(name) \
+#define DECLARE_uint32(name) extern uint32_t &FLAGS_##name;
-  extern std::reference_wrapper<uint32_t> FLAGS_##name;
 #endif  // !DEFINE_uint32