cmake warning regarding gflags

```
$ cmake -G Ninja -DCMAKE_INSTALL_PREFIX=. -DLLVM_PATH=/android0/llvm-project/llvm/build ..
...
CMake Warning at third_party/glog/CMakeLists.txt:63 (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" (requested
version 2.2.0) 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.
```