gflags in cmake on linux

Summary:
We should use it if available otherwise the tools builds never work. Thanks to #3212, we can set -DGFLAGS=1 and it'll be independent of the namespace with which gflags was compiled.
Closes https://github.com/facebook/rocksdb/pull/3214

Differential Revision: D6462214

Pulled By: ajkr

fbshipit-source-id: db4e5f1b905322e3119554a9d01b57532c499384

CMakeLists.txt

@@ -249,6 +249,13 @@ if(WITH_UBSAN)
   endif()
 endif()
 
+find_package(gflags)
+if(gflags_FOUND)
+  add_definitions(-DGFLAGS=1)
+  include_directories(${gflags_INCLUDE_DIR})
+  list(APPEND THIRDPARTY_LIBS ${gflags_LIBRARIES})
+endif()
+
 find_package(NUMA)
 if(NUMA_FOUND)
   add_definitions(-DNUMA)