gnu: Add proper structs for fpos_t and fpos64_t

(backport <#4345>)
(cherry picked from commit 872642a)

Author: snogge

src/unix/linux_like/linux/gnu/mod.rs

@@ -407,6 +407,24 @@ s! {
         #[cfg(target_pointer_width = "64")]
         __size: [c_char; 32],
     }
+
+    pub struct mbstate_t {
+        __count: c_int,
+        __wchb: [c_char; 4],
+    }
+
+    pub struct fpos64_t {
+        __pos: off64_t,
+        __state: crate::mbstate_t,
+    }
+
+    pub struct fpos_t {
+        #[cfg(not(gnu_file_offset_bits64))]
+        __pos: off_t,
+        #[cfg(gnu_file_offset_bits64)]
+        __pos: off64_t,
+        __state: crate::mbstate_t,
+    }
 }
 
 impl siginfo_t {

src/unix/linux_like/linux/mod.rs

@@ -74,9 +74,14 @@ pub type iconv_t = *mut c_void;
 pub type sctp_assoc_t = __s32;
 
 pub type eventfd_t = u64;
-missing! {
-    #[cfg_attr(feature = "extra_traits", derive(Debug))]
-    pub enum fpos64_t {} // FIXME(linux): fill this out with a struct
+
+cfg_if! {
+    if #[cfg(not(target_env = "gnu"))] {
+        missing! {
+            #[cfg_attr(feature = "extra_traits", derive(Debug))]
+            pub enum fpos64_t {} // FIXME(linux): fill this out with a struct
+        }
+    }
 }
 
 e! {

src/unix/mod.rs

@@ -539,11 +539,18 @@ cfg_if! {
     }
 }
 
+cfg_if! {
+    if #[cfg(not(target_env = "gnu"))] {
+        missing! {
+            #[cfg_attr(feature = "extra_traits", derive(Debug))]
+            pub enum fpos_t {} // FIXME(unix): fill this out with a struct
+        }
+    }
+}
+
 missing! {
     #[cfg_attr(feature = "extra_traits", derive(Debug))]
     pub enum FILE {}
-    #[cfg_attr(feature = "extra_traits", derive(Debug))]
-    pub enum fpos_t {} // FIXME(unix): fill this out with a struct
 }
 
 extern "C" {