Add static indicating NT vs 9x/ME

This will be used/needed for some APIs that exist on both platforms but with different behavior/capabilities.

Author: seritools

library/std/src/sys/windows/c.rs

@@ -721,4 +721,5 @@ extern "system" {
 
     pub fn GetTickCount() -> DWORD;
     pub fn GetCurrentThreadId() -> DWORD;
+    pub fn GetVersion() -> DWORD;
 }

library/std/src/sys/windows/compat.rs

@@ -24,6 +24,8 @@ use crate::ptr::NonNull;
 use crate::sync::atomic::Ordering;
 use crate::sys::c;
 
+pub(crate) mod version;
+
 // This uses a static initializer to preload some imported functions.
 // The CRT (C runtime) executes static initializers before `main`
 // is called (for binaries) and before `DllMain` is called (for DLLs).
@@ -65,6 +67,8 @@ unsafe extern "C" fn init() {
 
     // Attempt to preload the synch functions.
     load_synch_functions();
+    // Determine if running on 9x or NT
+    version::init();
 }
 
 /// Helper macro for creating CStrs from literals and symbol names.

library/std/src/sys/windows/compat/version.rs

@@ -0,0 +1,15 @@
+use crate::sys::c;
+
+static mut IS_NT: bool = true;
+
+pub(super) unsafe extern "C" fn init() {
+    // according to old MSDN info, the high-order bit is set only on 95/98/ME.
+    IS_NT = c::GetVersion() < 0x8000_0000;
+}
+
+/// Returns true if we are running on a Windows NT-based system. Only use this for APIs where the
+/// same API differs in behavior or capability on 9x/ME compared to NT.
+#[inline(always)]
+pub(crate) fn is_windows_nt() -> bool {
+    unsafe { IS_NT }
+}