find_package(gazebo) broken on Windows

[traversaro]

The PR #2864 , in particular https://github.com/osrf/gazebo/pull/2864/files#diff-0838b3a1790a01bd4c5b10e6789d5b7309df76869413ea9e166ee36815653b70R110, broke the calls to find_package(gazebo) on Windows. The error that you get as soon as you call find_package(gazebo) is:

CMake Error at C:/Users/STraversaro/Miniforge3/envs/robsup/Library/lib/cmake/gazebo/gazebo-config.cmake:146 (message):
  Library 'gazebo' in package GAZEBO is not installed properly
Call Stack (most recent call first):
  cmake/BuildGazeboYARPPlugins.cmake:31 (find_package)
  build-conda3/install/share/YCM/modules/FindOrBuildPackage.cmake:215 (include)
  cmake/RobotologySuperbuildLogic.cmake:12 (find_or_build_package)
  CMakeLists.txt:45 (include)

This is because the line in https://github.com/osrf/gazebo/blob/72f40624d4ef33166c8cb864390d6a1769044b14/CMakeLists.txt#L438 should be modified in Windows to read libgazebo instead of gazebo .