Use /MT instead of /MD when compiling libffi on Windows.
Use /MT instead of /MD when compiling libffi on Windows.

Otherwise you get mismatches and crazy build errors like:
  https://gist.github.com/575c1a006eee25cf5510

unbornchikken/node-ffi,craigcomstocks/node-ffi,lzpfmh/node-ffi,paulcbetts/node-ffi,node-ffi/node-ffi,skoky/node-ffi,skoky/node-ffi,craigcomstocks/node-ffi,paulcbetts/node-ffi,craigcomstocks/node-ffi,node-ffi/node-ffi,pmuellr/node-ffi,paulcbetts/node-ffi,lzpfmh/node-ffi,unbornchikken/node-ffi,lzpfmh/node-ffi,node-ffi/node-ffi,Icenium/node-ffi,node-ffi/node-ffi,lzpfmh/node-ffi,skoky/node-ffi,unrealsolver/node-ffi,pmuellr/node-ffi,unbornchikken/node-ffi,craigcomstocks/node-ffi,skoky/node-ffi,unrealsolver/node-ffi,Icenium/node-ffi,paulcbetts/node-ffi,unbornchikken/node-ffi,Icenium/node-ffi,pmuellr/node-ffi,pmuellr/node-ffi,unrealsolver/node-ffi,unrealsolver/node-ffi,Icenium/node-ffi