Fix build with GCC 4.9+.
    
    GCC 4.9+ is too aggressive when optimizing functions with nonnull
    attributes. This removes most of asserts() in the plugin.
    GCC 6 adds warnings for these cases.
    
    We are disabling the unwanted condition pruning by adding
    -fno-delete-null-pointer-checks argument.
    BIND 9 did the same in its commit 603a78708343f063b44affb882ef93bb19a5142a.
    
    Additionally we enable nonnull attribute only when the build is running under
    Clang static analyzer or Coverity.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1307346