Add a test that the const extern fn macro works

By default, any functions defined in this macro (such as `CMSG_SPACE`)
should be `const`.

tests/const_fn.rs

@@ -0,0 +1,3 @@
+#[cfg(target_os = "linux")]
+const _FOO: libc::c_uint = unsafe { libc::CMSG_SPACE(1) };
+//^ if CMSG_SPACE is not const, this will fail to compile