Make glog depend on gflags

cmake/external/glog.cmake

@@ -20,6 +20,10 @@ ExternalProject_Add(Glog_project
         -DCMAKE_VERBOSE_MAKEFILE:BOOL=OFF
         -DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=ON)
 
+IF(NOT gflags_FOUND)
+    ADD_DEPENDENCIES(Glog_project gflags_project)
+ENDIF()
+
 ADD_LIBRARY(GLOG STATIC IMPORTED) 
 SET_PROPERTY(TARGET GLOG PROPERTY IMPORTED_LOCATION ${CMAKE_CURRENT_BINARY_DIR}/glog_project/src/glog/libglog.a)
 SET(GLOG_LIBRARIES "")