locally disable pragma warnings on gcc < 4.6

locally disable pragma warnings on gcc < 4.6

diagnostic push|pop are not present before 4.6, so each use gives a
warning. Temporarily ignore these by disabling/reenabling -Wpragmas.
Note that this means the other disabled warnings are not scoped on
old gcc. I don't think this is worth fixing, since newer gcc will
respect the scope and show us the warnings outside of it.

Signed-off-by: Josh Durgin <jdurgin@redhat.com>