Move -fno-delete-null-pointer-checks to optional

It is no longer supported on El Capitan's clang, which was causing my
cmake/osx/qt build to fail with:

    clang: error: optimization flag '-fno-delete-null-pointer-checks' is not
    supported

Change-Id: Idfac336758cdea9a34446d8281cd237ad3ee6823
Reviewed-on: https://code.wireshark.org/review/11571
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>