Mac: fix linking error

ld: warning: option -s is obsolete and being ignored
ld: internal error: atom not found in
symbolIndex(__ZNSt3__1lsINS_11char_traitsIcEEEERNS_13basic_ostreamIcT_EES6_PKc)
for architecture x86_64

It's funny clang says -s is being ignored, but removing -s fixed the
error.