Windows: Use UTF-8 locale when active code page is UTF-8

XZ Utils 5.6.3 set the active code page to UTF-8 to fix CVE-2024-47611.
This wasn't paired with UCRT-specific setlocale(LC_ALL, ".UTF8"), thus
non-ASCII characters from translations became mojibake.

Fixes: 46ee006162
(cherry picked from commit 0d0b574cc4)

src/common/tuklib_gettext.h

@@ -15,6 +15,34 @@
 #include "tuklib_common.h"
 #include <locale.h>
 
+#if defined(_WIN32) && !defined(__CYGWIN__)
+#	define WIN32_LEAN_AND_MEAN
+#	include <windows.h>
+	// To use UTF-8 code page on Windows 10 version 1903 and later, the
+	// active code page has to be set to UTF-8 in the application manifest
+	// and UCRT-specific setlocale(LC_ALL, ".UTF8") must be called. The
+	// manifest makes argv[] use UTF-8 (which setlocale() cannot affect)
+	// and the special setlocale() call makes mbrtowc() and such functions
+	// use UTF-8. (It's weird why regular setlocale(LC_ALL, "") doesn't
+	// use the code page from the application manifest.)
+	//
+	// The two things have quite a bit of overlap though. For example,
+	// both affect the code page used in the file system APIs. Thus,
+	// if argv[] isn't in UTF-8, using setlocale() to set UTF-8 will
+	// break non-ASCII filenames that have been passed as command line
+	// arguments. Thus, it's best to set an UTF-8 locale only when
+	// the active code page is UTF-8.
+	//
+	// NOTE: Only UCRT supports the UTF-8 locale string, thus this
+	// will fail with MSVCRT if the active code page is UTF-8. That
+	// shouldn't be too bad because UTF-8 doesn't work properly with
+	// MSVCRT anyway.
+#	define tuklib_gettext_setlocale() \
+		setlocale(LC_ALL, GetACP() == CP_UTF8 ? ".UTF8" : "")
+#else
+#	define tuklib_gettext_setlocale() setlocale(LC_ALL, "")
+#endif
+
 #ifndef TUKLIB_GETTEXT
 #	ifdef ENABLE_NLS
 #		define TUKLIB_GETTEXT 1
@@ -27,14 +55,14 @@
 #	include <libintl.h>
 #	define tuklib_gettext_init(package, localedir) \
 		do { \
-			setlocale(LC_ALL, ""); \
+			tuklib_gettext_setlocale(); \
 			bindtextdomain(package, localedir); \
 			textdomain(package); \
 		} while (0)
 #	define _(msgid) gettext(msgid)
 #else
 #	define tuklib_gettext_init(package, localedir) \
-		setlocale(LC_ALL, "")
+		tuklib_gettext_setlocale()
 #	define _(msgid) (msgid)
 #	define ngettext(msgid1, msgid2, n) ((n) == 1 ? (msgid1) : (msgid2))
 #endif