Add option to encfs formula to build with external OSXFUSE.

I know that this is going to be controversial, but before rejecting this pull request, please consider the exceptional (and hopefully temporary) status of OS X Fuse projects:

- fuse4x is essentially dead, being merged with OSXFUSE.
- nobody wants to build a formula for OSXFUSE as long as their build system is not updated.
- OSXFUSE is the way to go for obvious reasons, being an active project: besides, it allows a user to install other software (e.g., TrueCrypt), which is not possible with fuse4x.
- fuse4x and OSXFUSE cannot (easily) coexist.
- See also https://github.com/mxcl/homebrew/issues/17274.

That said, with this pull request fuse4x still is the default and recommended way to install encfs.

Acknowledgments: the code is adapted from https://github.com/jollyjinx/encfs.macosx. (That code also contains a patch for encfs, which looks reasonable to me, although I do not know which “problems” jollyjinx refers to precisely.)