Disable externalwrench plugin if Gazebo version < 8.0 (#434)

* disable extwrench plugin is gazebo version < 8.0

CHANGELOG.md

@@ -3,6 +3,10 @@ All notable changes to this project will be documented in this file.
 
 The format of this document is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## [Unreleased]
+### Fixed 
+- Fixed compilation on Gazebo 7 by disabling compilation of `externalwrench` plugin if the project is configured with Gazebo 7 (https://github.com/robotology/gazebo-yarp-plugins/pull/434).
+
 ## [3.2.0] - 2019-07-01
 ### Added
 - Add `resetSimulation` method to reset simulation state and time time, in the `gazebo_yarp_clock` RPC interface (https://github.com/robotology/gazebo-yarp-plugins/pull/345).

plugins/CMakeLists.txt

@@ -1,7 +1,12 @@
 add_subdirectory(camera)
 add_subdirectory(clock)
 add_subdirectory(controlboard)
-add_subdirectory(externalwrench)
+
+# Disable externalwrench plugin if Gazebo version is < 8.0
+if (NOT GAZEBO_VERSION VERSION_LESS 8.0)
+    add_subdirectory(externalwrench)
+endif()
+
 add_subdirectory(fakecontrolboard)
 add_subdirectory(forcetorque)
 add_subdirectory(imu)
@@ -37,4 +42,4 @@ add_subdirectory(depthCamera)
 add_subdirectory(contactloadcellarray)
 add_subdirectory(modelposepublisher)
 add_subdirectory(basestate)
-add_subdirectory(configurationoverride)
+add_subdirectory(configurationoverride)