Revert "scons: Fix build when rtti is disabled"

This reverts commit 94d05bf87a21bd364e84f699a0064e5fba58a6f9 as it has a
few problems:

- it breaks windows builds becuase env[LLVM_CXXFLAGS] is never set there

- it is merging not only rtti, but the whole cxxflags (defines etc)
  which has proven to be a source of troubles (breaks debugging etc.)
3 files changed