Try to use standard way to import gflags.

* See [here](https://gflags.github.io/gflags/#cmake).

Author: reyoung

CMakeLists.txt

@@ -26,7 +26,7 @@ find_package(NumPy REQUIRED)
 find_package(Threads REQUIRED)
 find_package(AVX QUIET)
 find_package(Glog REQUIRED)
-find_package(Gflags REQUIRED)
+find_package(gflags REQUIRED)
 find_package(GTest)
 find_package(Sphinx)
 find_package(Doxygen)