Correct the RPATH for fix location installations (gnuinstall)

cmake/modules/RootBuildOptions.cmake

@@ -258,7 +258,7 @@ set(CMAKE_MACOSX_RPATH TRUE)              # use RPATH for MacOSX
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE) # point to directories outside the build tree to the install RPATH
 
 # Check whether to add RPATH to the installation (the build tree always has the RPATH enabled)
-if(rpath)
+if(rpath OR gnuinstall)
   set(CMAKE_INSTALL_RPATH ${CMAKE_INSTALL_FULL_LIBDIR}) # install LIBDIR
   set(CMAKE_SKIP_INSTALL_RPATH FALSE)          # don't skip the full RPATH for the install tree
 elseif(APPLE)