Changed SeekData and SeekHole hardcoded values to libc ones and

added availability only to linux freebsd and dragonflybsd.

Author: ncaracci

src/unistd.rs

@@ -750,12 +750,14 @@ pub enum Whence {
     /// Specify an offset relative to the next location in the file greater than or 
     /// equal to offset that contains some data. If offset points to
     /// some data, then the file offset is set to offset.
+    #[cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd"))]
     SeekData,
     /// Specify an offset relative to the next hole in the file greater than
     /// or equal to offset. If offset points into the middle of a hole, then 
     /// the file offset should be set to offset. If there is no hole past offset,
     /// then the file offset should be adjusted to the end of the file (i.e., there
     /// is an implicit hole at the end of any file).
+    #[cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd"))]
     SeekHole
 }
 
@@ -765,8 +767,10 @@ impl Whence {
             &Whence::SeekSet => libc::SEEK_SET,
             &Whence::SeekCur => libc::SEEK_CUR,
             &Whence::SeekEnd => libc::SEEK_END,
-            &Whence::SeekData => 3,
-            &Whence::SeekHole => 4
+            #[cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd"))]
+            &Whence::SeekData => libc::SEEK_DATA,
+            #[cfg(any(target_os = "linux", target_os = "dragonfly", target_os = "freebsd"))]
+            &Whence::SeekHole => libc::SEEK_HOLE
         }
     }