cmake: use gflags package config instead of a find module

This allows to avoid conflicts between third party gflags find modules floating
around. If a package provide a local version of gflags find module, clients
which locate glog will incorrectly use it. Another problem is a CMake error message
"cmake_policy PUSH without matching POP" in nested find_package calls. In summary,
we need to ensure to use the original gflags package config.

CMakeLists.txt

@@ -599,8 +599,19 @@ install (TARGETS glog
   LIBRARY DESTINATION ${_glog_CMake_LIBDIR}
   ARCHIVE DESTINATION ${_glog_CMake_LIBDIR})
 
+set (glog_POLICY_VERSION 3.0)
+
 if (gflags_FOUND)
-  set (gflags_DEPENDENCY "find_dependency (gflags ${gflags_VERSION})")
+  # Ensure clients locate only the package config and not third party find
+  # modules having the same name. This avoid cmake_policy PUSH/POP errors.
+  if (CMAKE_VERSION VERSION_LESS 3.9)
+    set (gflags_DEPENDENCY "find_dependency (gflags ${gflags_VERSION})")
+  else (CMAKE_VERSION VERSION_LESS 3.9)
+    # Passing additional find_package arguments to find_dependency is possible
+    # starting with CMake 3.9.
+    set (glog_POLICY_VERSION 3.9)
+    set (gflags_DEPENDENCY "find_dependency (gflags ${gflags_VERSION} NO_MODULE)")
+  endif (CMAKE_VERSION VERSION_LESS 3.9)
 endif (gflags_FOUND)
 
 configure_package_config_file (glog-config.cmake.in

glog-config.cmake.in

 cmake_policy (PUSH)
-cmake_policy (VERSION 3.0)
+cmake_policy (VERSION @glog_POLICY_VERSION@)
 
 @PACKAGE_INIT@