avoid Mac OS X ranlib error (#1007)
#992
[ 49%] Linking CXX static library libwabt.a
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: libwabt.a(config.cc.o) has no symbols
/Library/Developer/CommandLineTools/usr/bin/ranlib: file: libwabt.a(config.cc.o) has no symbols
If I add a dummy function for clang, there's no ranlib error.
