Use the rust-bindgen crate properly

Using the changes from @gadomski along with some additional work, to get the
generated bindings working again. Works on macOS and FreeBSD 11. A couple of
hacks are needed for FreeBSD, but nothing too serious.

Changed to use the libc prefix, and changed to use the generated enums.

Fixes #22, #15, and #14

cargo test passes

src/conversions.rs

@@ -22,9 +22,9 @@ pub trait FromRust<T> {
 impl FromRust<bool> for bindings::MagickBooleanType {
     fn from_rust(b: bool) -> Self {
         if b {
-            bindings::MagickTrue
+            bindings::MagickBooleanType::MagickTrue
         } else {
-            bindings::MagickFalse
+            bindings::MagickBooleanType::MagickFalse
         }
     }
 }