Work around 'unreferenced function' warnings in NO_STL builds

Normally, as_utf8_begin et al are used in STL functions but when STL is
disabled, these are only used if the target platform lacks first class
support for wchar_t FILE* APIs. With some warning levels we consequently
can get warnings about these functions not being referenced.

Not defining these in the first place is difficult because of the
complexity of the selection logic for open_file_wide so for now just
mark these as unused. The strange (void)& syntax is needed for MSVC to
not trigger another warning...

The workaround is narrowly scoped to avoid unforeseen compatibility issues.

Fixes #619.

Author: zeux

src/pugixml.cpp

@@ -5054,6 +5054,14 @@ PUGI_IMPL_NS_BEGIN
 #if defined(PUGI_IMPL_MSVC_CRT_VERSION) || defined(__BORLANDC__) || (defined(__MINGW32__) && (!defined(__STRICT_ANSI__) || defined(__MINGW64_VERSION_MAJOR)))
 	PUGI_IMPL_FN FILE* open_file_wide(const wchar_t* path, const wchar_t* mode)
 	{
+#ifdef PUGIXML_NO_STL
+		// ensure these symbols are consistently referenced to avoid 'unreferenced function' warnings
+		// note that generally these functions are used in STL builds, but PUGIXML_NO_STL leaves the only usage in convert_path_heap
+		(void)&as_utf8_begin;
+		(void)&as_utf8_end;
+		(void)&strlength_wide;
+#endif
+
 #if defined(PUGI_IMPL_MSVC_CRT_VERSION) && PUGI_IMPL_MSVC_CRT_VERSION >= 1400
 		FILE* file = NULL;
 		return _wfopen_s(&file, path, mode) == 0 ? file : NULL;