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

build.rs

@@ -1,28 +1,77 @@
+/*
+ * Copyright 2016-2017 Nathan Fiedler
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 extern crate bindgen;
 extern crate pkg_config;
 
 use std::env;
-use std::path::PathBuf;
+use std::fs::File;
+use std::io::prelude::*;
+use std::path::{Path, PathBuf};
 
 const MIN_VERSION: &'static str = "6.9";
 const MAX_VERSION: &'static str = "6.10";
 
+static HEADER: &'static str = "#include <wand/MagickWand.h>\n";
+
 fn main() {
+    // Assert that the appropriate version of MagickWand is installed,
+    // since we are very dependent on the particulars of MagickWand.
     let library = pkg_config::Config::new()
         .atleast_version(MIN_VERSION)
         .arg(format!("--max-version={}", MAX_VERSION))
         .probe("MagickWand")
         .unwrap();
-    let mut builder = bindgen::Builder::default()
-        .no_unstable_rust()
-        .emit_builtins()
-        .ctypes_prefix("libc")
-        .raw_line("extern crate libc;")
-        .header("wrapper.h");
-    for include_path in library.include_paths {
-        builder = builder.clang_arg(format!("-I{}", include_path.to_string_lossy()));
+
+    // If the generated bindings are missing, generate them now.
+    let out_dir = PathBuf::from(env::var("OUT_DIR").unwrap());
+    let bindings_path_str = out_dir.join("bindings.rs");
+    if !Path::new(&bindings_path_str).exists() {
+
+        // Create the header file that rust-bindgen needs as input.
+        let gen_h_path = out_dir.join("gen.h");
+        let mut gen_h = File::create(&gen_h_path).expect("could not create file");
+        gen_h.write_all(HEADER.as_bytes()).expect("could not write header file");
+
+        // Geneate the bindings.
+        let mut builder = bindgen::Builder::default()
+            .no_unstable_rust()
+            .emit_builtins()
+            .ctypes_prefix("libc")
+            .raw_line("extern crate libc;")
+            .header(gen_h_path.to_str().unwrap());
+        for include_path in library.include_paths {
+            builder = builder.clang_arg(format!("-I{}", include_path.to_string_lossy()));
+        }
+        if cfg!(target_os = "freebsd") {
+            // pkg_config does not seem to work properly on FreeBSD, so
+            // hard-code the builder settings for the time being.
+            builder = builder.clang_arg("-I/usr/local/include/ImageMagick-6");
+            // Need to hack the linker flags as well.
+            println!("cargo:rustc-link-lib=dylib=MagickWand-6");
+            println!("cargo:rustc-link-search=native=/usr/local/lib");
+        }
+        let bindings = builder.generate().unwrap();
+        let mut file = File::create(&bindings_path_str).expect("could not create bindings file");
+        // Work around the include! issue in rustc (as described in the
+        // rust-bindgen README file) by wrapping the generated code in a
+        // `pub mod` declaration; see issue #359 in (old) rust-bindgen.
+        file.write(b"pub mod bindings {\n").unwrap();
+        file.write(bindings.to_string().as_bytes()).unwrap();
+        file.write(b"\n}").unwrap();
+
+        std::fs::remove_file(&gen_h_path).expect("could not remove header file");
     }
-    let bindings = builder.generate().unwrap();
-    let outfile = PathBuf::from(env::var("OUT_DIR").unwrap());
-    bindings.write_to_file(outfile.join("bindings.rs")).unwrap();
 }