v4l2-sys: Only use FreeBSD header include if the (cross-compilation) …

…target OS is FreeBSD

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`.

v4l2-sys/build.rs

@@ -4,7 +4,10 @@ use std::env;
 use std::path::{Path, PathBuf};
 
 fn main() {
-    let extra_include_paths = if cfg!(target_os = "freebsd") {
+    let extra_include_paths = if std::env::var("CARGO_CFG_TARGET_OS").unwrap() == "freebsd" {
+        // XXX: This path is wrong if the host OS != cross-compilation OS. If anything the host
+        // should simply set up the right path using `BINDGEN_EXTRA_CLANG_ARGS_<triple>`, allowing
+        // us to remove this workaround instead.
         assert!(
             Path::new("/usr/local/include/linux/videodev2.h").exists(),
             "Video4Linux `videodev2.h` UAPI header is required to generate bindings \