Remove --enable-shared=no from httrack.rb, fixing #19292 for OS X 10.7 x86_64 and 10.8 x86_64

On Lion (10.7) and Mountain Lion (10.8) the config argument `--enable-shared=no` will result in `ld: symbol(s) not found for architecture x86_64` error.
See https://github.com/mxcl/homebrew/issues/19292#issuecomment-17066392