Write assets like manual pages to a predictable location.

- Fixes #45.

README.md

@@ -29,4 +29,7 @@ cargo build
 ```
 
 The above creates the `sq` executable, the manual pages, and its shell
-completions.
+completions.  By default, the manual pages and shell completions are
+put into the `cargo` target directory, but the exact location is
+unpredictable.  To write the assets to a predictable location, set the
+environment variable `ASSET_OUT_DIR` to a suitable location.

build.rs

@@ -30,11 +30,21 @@ fn main() {
     generate_man_pages(&sq).unwrap();
 }
 
+/// Variable name to control the asset out directory with.
+const ASSET_OUT_DIR: &str = "ASSET_OUT_DIR";
+
 /// Returns the directory to write the given assets to.
 fn asset_out_dir(asset: &str) -> Result<PathBuf> {
+    println!("cargo:rerun-if-env-changed={}", ASSET_OUT_DIR);
     let outdir: PathBuf =
-        env::var_os("OUT_DIR").expect("OUT_DIR not set").into();
-    assert!(outdir.is_dir());
+        env::var_os(ASSET_OUT_DIR).unwrap_or_else(
+            || env::var_os("OUT_DIR").expect("OUT_DIR not set")).into();
+    if outdir.exists() && ! outdir.is_dir() {
+        return Err(
+            anyhow::anyhow!("{}={:?} is not a directory",
+                            ASSET_OUT_DIR, outdir));
+    }
+
     let path = outdir.join(asset);
     fs::create_dir_all(&path)?;
     Ok(path)