Older versions of Xcode don't support -Wunused-but-set-variable, so
remove it from the list of options with which to compile SpanDSP.
Change-Id: Idd298e19eaf770980da179412480e17a00514d36
Reviewed-on: https://code.wireshark.org/review/22991
Reviewed-by: Guy Harris <guy@alum.mit.edu>