cmake: disable gflags support in rocksdb

to silence the warnings like

CMake Warning at CMakeLists.txt:73 (find_package):
  By not providing "Findgflags.cmake" in CMAKE_MODULE_PATH this project
has
  asked CMake to find a package configuration file provided by "gflags",
but
  CMake did not find one.

  Could not find a package configuration file provided by "gflags" with
any
  of the following names:

    gflagsConfig.cmake
    gflags-config.cmake

  Add the installation prefix of "gflags" to CMAKE_PREFIX_PATH or set
  "gflags_DIR" to a directory containing one of the above files.  If
"gflags"
  provides a separate development package or SDK, be sure it has been
  installed.

Signed-off-by: Kefu Chai <kchai@redhat.com>

cmake/modules/BuildRocksDB.cmake

@@ -1,7 +1,8 @@
 include(CheckCXXSourceRuns)
 
 function(do_build_rocksdb)
-    set(ROCKSDB_CMAKE_ARGS -DCMAKE_POSITION_INDEPENDENT_CODE=ON)
+  set(ROCKSDB_CMAKE_ARGS -DCMAKE_POSITION_INDEPENDENT_CODE=ON)
+  list(APPEND ROCKSDB_CMAKE_ARGS -DWITH_GFLAGS=OFF)
 
   if(ALLOCATOR STREQUAL "jemalloc")
     list(APPEND ROCKSDB_CMAKE_ARGS -DWITH_JEMALLOC=ON)