Use Protobuf_IMPORT_DIRS instead of PROTOBUF_IMPORT_DIRS for compatib…

…ility with Protobuf CMake config (#715)

Signed-off-by: Silvio Traversaro <silvio.traversaro@iit.it>

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.5.1 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.10.2 FATAL_ERROR)
 
 #============================================================================
 # Initialize the project
@@ -136,7 +136,7 @@ ign_find_package(IgnProtobuf
                  REQUIRED
                  COMPONENTS all
                  PRETTY Protobuf)
-set(PROTOBUF_IMPORT_DIRS ${ignition-msgs5_INCLUDE_DIRS})
+set(Protobuf_IMPORT_DIRS ${ignition-msgs5_INCLUDE_DIRS})
 
 # Plugin install dirs
 set(IGNITION_GAZEBO_PLUGIN_INSTALL_DIR