Add a linux_kernel cfg, to simplify a common platform set. (#675)

"android" and "linux" share much in common, so add a `linux_kernel`
config to factor out `any(target_os = "android", target_os = "linux")`.

build.rs

@@ -140,6 +140,12 @@ fn main() {
     {
         use_feature("bsd");
     }
+
+    // Add some additional common target combinations.
+    if target_os == "android" || target_os == "linux" {
+        use_feature("linux_kernel");
+    }
+
     if target_os == "wasi" {
         use_feature_or_nothing("wasi_ext");
     }

src/backend/libc/c.rs

@@ -1,9 +1,9 @@
 pub(crate) use libc::*;
 
 /// `PROC_SUPER_MAGIC`—The magic number for the procfs filesystem.
-#[cfg(all(any(target_os = "android", target_os = "linux"), target_env = "musl"))]
+#[cfg(all(linux_kernel, target_env = "musl"))]
 pub(crate) const PROC_SUPER_MAGIC: u32 = 0x0000_9fa0;
 
 /// `NFS_SUPER_MAGIC`—The magic number for the NFS filesystem.
-#[cfg(all(any(target_os = "android", target_os = "linux"), target_env = "musl"))]
+#[cfg(all(linux_kernel, target_env = "musl"))]
 pub(crate) const NFS_SUPER_MAGIC: u32 = 0x0000_6969;

src/backend/libc/conv.rs

@@ -107,7 +107,7 @@ pub(super) fn syscall_ret_usize(raw: c::c_long) -> io::Result<usize> {
     }
 }
 
-#[cfg(any(target_os = "android", target_os = "linux"))]
+#[cfg(linux_kernel)]
 #[inline]
 pub(super) fn syscall_ret_u32(raw: c::c_long) -> io::Result<u32> {
     if raw == -1 {

src/backend/libc/fs/mod.rs

@@ -1,6 +1,6 @@
 #[cfg(not(target_os = "redox"))]
 pub(crate) mod dir;
-#[cfg(any(target_os = "android", target_os = "linux"))]
+#[cfg(linux_kernel)]
 pub mod inotify;
 #[cfg(not(any(target_os = "haiku", target_os = "redox", target_os = "wasi")))]
 pub(crate) mod makedev;