Fix python meta data generation

Summary

Several smaller changes to improve python meta data handling, see dune-grid#141 (closed).

Main changes

• Move set(<module>_FOUND 1) to the end of dune_project()
• Add set(HAVE_<MODULE> 1) at the end of find_dune_project() to provide the "dune-way" of indicating found packages
• Collect all HAVE_<MODULE> flags for all dependencies in the python _cmake_flags variable

cmake/modules/DuneModuleDependencies.cmake

@@ -261,6 +261,8 @@ macro(find_dune_package module)
     endif()
   endif()
   set(DUNE_${module}_FOUND ${${module}_FOUND})
+  dune_module_to_uppercase(_upper_case "${module}")
+  set(HAVE_${_upper_case} "${${module}_FOUND}") # need for example for the python meta data...
 endmacro(find_dune_package module)