builtin: fix GetFinalPathNameByHandleW declaration for TCC on Windows

TCC''s bundled <windows.h> does not include <fileapi.h>, so
GetFinalPathNameByHandleW had no C declaration in scope when TCC
compiled any program that pulled in builtin (e.g. via os.real_path).
TCC (in C99 mode) treats implicit function declarations as hard errors.

Provide the declaration for TCC only, via a header that is #insert-ed
under $if windows and guarded with `#ifdef __TINYC__`:

- TCC: __TINYC__ is defined, so the extern is supplied. TCC''s own
  <fileapi.h> cannot be used instead because it pulls in <apiset.h>,
  a header TCC does not ship.
- GCC/MSVC: __TINYC__ is not defined, so the extern is skipped and the
  call resolves against the SDK header that <windows.h> pulls in
  transitively. Emitting a V-side extern here would conflict with the
  SDK''s `DWORD WINAPI` signature (DWORD is unsigned long, not u32).

Gating in the C preprocessor (rather than a comptime `$if tinyc`) is
deliberate: it keeps the guard intact under `-cross`, where V emits all
declarations unconditionally. The same `#ifdef __TINYC__` idiom is
already used for TCC/Windows in vlib/v/gen/c/cheaders.v.

Verified on Windows: generated C wraps the extern in `#ifdef __TINYC__`
for both normal and -cross GCC builds (gcc compiles both with no
conflict), and `vlib/os/` tests pass (24/24) with GCC.

Co-Authored-By: WOZCODE <contact@withwoz.com>

Author: PythonWillRule

vlib/builtin/cfns.c.v

@@ -295,7 +295,13 @@ fn C.CreateFile(lpFilename &u16, dwDesiredAccess u32, dwShareMode u32, lpSecurit
 fn C.CreateFileW(lpFilename &u16, dwDesiredAccess u32, dwShareMode u32, lpSecurityAttributes &u16, dwCreationDisposition u32,
 	dwFlagsAndAttributes u32, hTemplateFile voidptr) voidptr
 
-fn C.GetFinalPathNameByHandleW(hFile voidptr, lpFilePath &u16, nSize u32, dwFlags u32) u32
+$if windows {
+	// The TCC-only declaration is guarded with `#ifdef __TINYC__` in the
+	// header, so it survives cross compilation; GCC/MSVC use the SDK header.
+	#insert "@VEXEROOT/vlib/builtin/cfns_windows_tcc.h"
+
+	fn C.GetFinalPathNameByHandleW(hFile voidptr, lpFilePath &u16, nSize u32, dwFlags u32) u32
+}
 
 fn C.CreatePipe(hReadPipe &voidptr, hWritePipe &voidptr, lpPipeAttributes voidptr, nSize u32) bool
 

vlib/builtin/cfns_windows_tcc.h

@@ -0,0 +1,15 @@
+// TCC's bundled <windows.h> does not include <fileapi.h>, and TCC's own
+// <fileapi.h> cannot be used either because it pulls in <apiset.h>, a header
+// TCC does not ship. As a result GetFinalPathNameByHandleW has no declaration
+// in scope when TCC compiles code that uses it (e.g. os.real_path).
+//
+// Provide the declaration for TCC only. GCC/MSVC get it from the Windows SDK
+// (via <windows.h> -> <fileapi.h>); emitting a V-side extern there would
+// conflict with the SDK's `DWORD WINAPI` signature, since DWORD is
+// `unsigned long`, not `unsigned int`.
+#ifdef __TINYC__
+#ifndef GetFinalPathNameByHandleW
+extern unsigned int GetFinalPathNameByHandleW(void *hFile, unsigned short *lpFilePath,
+	unsigned int nSize, unsigned int dwFlags);
+#endif
+#endif