[bugfix][cmake] Fix installation path for cmake modules

The current path installs to: ROOT/share/cmake/modules
It should be: ROOT/share/dune/cmake/modules

cmake/modules/CMakeLists.txt

@@ -7,4 +7,4 @@ set(modules
     FindSIONlib.cmake
     FindZOLTAN.cmake
 )
-install(FILES ${modules} DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/cmake/modules)
+install(FILES ${modules} DESTINATION ${DUNE_INSTALL_MODULEDIR})