Set cmake_plugin to absolute path in pc file.

CMakeLists.txt

@@ -99,7 +99,7 @@ IF(NOT CLIENT_ONLY)
   INSTALL(FILES cmake-modules/gepetto-viewer-corba-plugin.cmake
     DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/${PROJECT_NAME}/cmake/
     PERMISSIONS OWNER_READ GROUP_READ WORLD_READ OWNER_WRITE)
-  SET(PKG_CONFIG_EXTRA "${PKG_CONFIG_EXTRA}\ncmake_plugin=${CMAKE_INSTALL_DATAROOTDIR}/${PROJECT_NAME}/cmake/gepetto-viewer-corba-plugin.cmake")
+  SET(PKG_CONFIG_EXTRA "${PKG_CONFIG_EXTRA}\ncmake_plugin=${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_DATAROOTDIR}/${PROJECT_NAME}/cmake/gepetto-viewer-corba-plugin.cmake")
 
   INCLUDE_DIRECTORIES(SYSTEM ${EIGEN3_INCLUDE_DIRS} ${QT_INCLUDE_DIRS})