Add DECLARE_uint32 to gflags compatibility (#10729)

Summary:
Older versions of gflags do not have `DEFINE_uint32` and `DECLARE_uint32`. In util/gflag_compat.h, we already add a hack for `DEFINE_uint32`. This PR adds a hack for `DECLARE_uint32`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10729

Test Plan:
ROCKSDB_NO_FBCODE=1 make V=1 -j16 db_stress
make check

Resolves https://github.com/facebook/rocksdb/issues/10704

Reviewed By: pdillinger

Differential Revision: D39789183

Pulled By: riversand963

fbshipit-source-id: a58747e0163dcf55dd762733aa5c40d8f0ae70a6

util/gflags_compat.h

@@ -6,6 +6,8 @@
 #pragma once
 #include <gflags/gflags.h>
 
+#include <functional>
+
 #ifndef GFLAGS_NAMESPACE
 // in case it's not defined in old versions, that's probably because it was
 // still google by default.
@@ -20,6 +22,9 @@
   namespace gflags_compat {                       \
   DEFINE_int32(name, val, txt);                   \
   }                                               \
-  uint32_t &FLAGS_##name =            \
-      *reinterpret_cast<uint32_t *>(&gflags_compat::FLAGS_##name);
-#endif
+  std::reference_wrapper<uint32_t> FLAGS_##name = \
+      std::ref(*reinterpret_cast<uint32_t *>(&gflags_compat::FLAGS_##name));
+
+#define DECLARE_uint32(name) \
+  extern std::reference_wrapper<uint32_t> FLAGS_##name;
+#endif  // !DEFINE_uint32