clang does not know fno-delete-null-pointer-checks

as introduced in https://github.com/RIOT-OS/RIOT/pull/628 the parameter "-fno-delete-null-pointer-checks" is handed over to the compiler.

clang throws a warning and ignores it.

task: try to find a way to get the desired functionality in clang, and or try to find a way to get rid of this warning.