v4l2-sys: Replace FreeBSD **host-only** include path override with docs

With `cfg!()` on `target_os` this include path is unconditionally used
if the _host_ OS is FreeBSD, even if the target OS is different (and its
cross-compilation headers are installed elsewhere on the system).  The
accurate target OS, regardless of what the build script is _running on_
is stored in `CARGO_CFG_TARGET_OS`.

Since it is unlikely that the FreeBSD headers reside in `/usr/
local/include` when the *target* is FreeBSD while the host may be
something completely different, remove the workaround and document
how the user can set up arbitrary include directories for their target
using `BINDGEN_EXTRA_CLANG_ARGS` (or the triple-specific variant) by
documenting this environment variable in the main `README`.  It is
common for developers to maintain such a configuration in their home
directory's `~/.cargo/ config.toml` for the various architectures that
they cross-compile to (together with related variables for the linker
and `cc-rs`).

README.md

@@ -94,3 +94,21 @@ fn main() {
 ```
 
 Have a look at the provided `examples` for more sample applications.
+
+### Building and cross-compiling
+
+When building on targets like FreeBSD, or cross-compiling for different targets entirely (as identified by their _target triple_), bindgen may not know where to find the headers if they are located in a nonstandard directory like `/usr/local/include`, resulting in an error similar to `wrapper.h:1:10: fatal error: 'linux/videodev2.h' file not found`.  In this case, provide the system include directory with the `-I` flag using the [target-specific environment variable][bindgen-env] (note that `-` is typically substituted with `_` to help shells like `bash` parse it successfully):
+
+```console
+$ BINDGEN_EXTRA_CLANG_ARGS_x86_64_unknown_freebsd="-I/usr/local/include" cargo build --target x86_64-unknown-freebsd
+```
+
+It is also possible to set this environment variable for Rust inside [`.cargo/config.toml`][cargo-config] in your project directory or user home directory:
+
+```toml
+[env]
+BINDGEN_EXTRA_CLANG_ARGS_x86_64-unknown-freebsd = "-I/usr/local/include"
+```
+
+[bindgen-env]: https://github.com/rust-lang/rust-bindgen/blob/main/README.md#environment-variables
+[cargo-config]: https://doc.rust-lang.org/cargo/reference/config.html

v4l2-sys/build.rs

@@ -1,24 +1,11 @@
 extern crate bindgen;
 
 use std::env;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 
 fn main() {
-    let extra_include_paths = if cfg!(target_os = "freebsd") {
-        assert!(
-            Path::new("/usr/local/include/linux/videodev2.h").exists(),
-            "Video4Linux `videodev2.h` UAPI header is required to generate bindings \
-            against `libv4l2` and the header file is missing.\n\
-            Consider installing `multimedia/v4l_compat` FreeBSD package."
-        );
-        vec!["-I/usr/local/include"]
-    } else {
-        vec![]
-    };
-
     let bindings = bindgen::Builder::default()
         .header("wrapper.h")
-        .clang_args(extra_include_paths)
         .generate()
         .expect("Failed to generate bindings");