Use gflags ALIAS instead of ${gflags_XXX} vars

The gflags project updated their CMake config last year with a
`gflags` ALIAS target. This can be used instead of the legacy
`${gflags_LIBRARIES}` and `${gflags_INCLUDE_DIRS}` variables. It also
looks cleaner.

Fixes #198

CMakeLists.txt

@@ -397,8 +397,7 @@ if (WIN32 AND HAVE_SNPRINTF)
 endif (WIN32 AND HAVE_SNPRINTF)
 
 if (gflags_FOUND)
-  target_include_directories (glog PUBLIC $<BUILD_INTERFACE:${gflags_INCLUDE_DIR}>)
-  target_link_libraries (glog PUBLIC ${gflags_LIBRARIES})
+  target_link_libraries (glog PUBLIC gflags)
 
   if (NOT BUILD_SHARED_LIBS)
     # Don't use __declspec(dllexport|dllimport) if this is a static build