Use stat for gmx_is_file. Fix get_libdir with symlinks.

Use stat for gmx_is_file. Fix get_libdir with symlinks.

In commit 4e057440520c1 I used lstat instead of stat confusing the
two. The commit tried to fix the misidentification of directories.
There is no need to distinquish between symlinks and files (and
thus break it if a binary is a symlink).

Change-Id: Ie964e826729a35f11516fb3d002ebc01cbb7a564