Fix relative symlink creation

Within `post.py` there is a function called `check_symlinks()`. This function makes sure any symlinks to files within the build are relative. If it thinks they are not relative, it unlinks the symlink and replaces it with a new relative version. I believe the change introduced in https://github.com/conda/conda-build/commit/37a0d8ab2bca4b3b4cec72cb34099e5b1c6ec68c contains an error. To illustrate the problem consider a symlink to another file in the same directory:
```
bob@localhost ~/tmp/linkfun/a/b/c $ ll
total 8
drwxr-xr-x  4 bob  users   136B 24 Dec 13:57 ./
drwxr-xr-x  3 bob  users   102B 24 Dec 13:30 ../
-rw-r--r--  1 bob  users     0B 24 Dec 11:02 file.dat
lrwxr-xr-x  1 bob  users    49B 24 Dec 11:04 filelink@ -> /Users/bob/tmp/linkfun/a/b/c/file.dat
```
The code uses `relpath(real_link_path, path)` to set the new "relative" `src`. Here:
```
from os.path import realpath, relpath
path = '/Users/bob/tmp/linkfun/a/b/c/filelink'
real_link_path = realpath(path)
print real_link_path
'/Users/bob/tmp/linkfun/a/b/c/file.dat'
src = relpath(real_link_path, path)
```
resulting in:
```
src = '../file.dat'
```
This is incorrect as it has added an unwanted `../`, resulting in a broken link. The problem is the start of the `relpath` should be the directory not the file. This PR fixes this issue.