Fix matcaffe build on OS X with XCode 6

Fixes `vecLib` and `boost`/`string` issues in https://github.com/BVLC/caffe/issues/1212

The explicit reference to `/usr/lib/libstdc++.dylib` in the Makefile is potentially brittle if `libstdc++.dylib` is installed to a non-standard location; perhaps there's a more elegant way to make this change?

Tested on:

## Mac

* OS X 10.9.5
* XCode 6.0.1
* Matlab R2014a

```
$pkgutil --pkg-info=com.apple.pkg.CLTools_Executables
package-id: com.apple.pkg.CLTools_Executables
version: 6.0.0.0.1.1410400753
volume: /
location: /
install-time: 1412278538
groups: com.apple.FindSystemFiles.pkg-group com.apple.DevToolsBoth.pkg-group com.apple.DevToolsNonRelocatableShared.pkg-group
```

## Ubuntu

* Ubuntu 14.04
* g++ 4.6
* Matlab R2014a