Vigra pollutes builds with pragmas that suppress warnings

[samkhal]

config.hxx currently uses compiler-specific warning suppression pragmas, such as here. This pollutes the build of anyone including this file who relies on the suppressed warnings, particularly for anyone also using -Werror, as this introduces compile errors that are dependent on where vigra is first included.

If suppressing these warnings from within vigra is desired, push and pop pragmas should be used to apply and reset these suppressions when entering and leaving vigra code.