tuklib_mbstr_width: Change the behavior when wcwidth() is not available

If wcwidth() isn't available (Windows), previously it was assumed
that one byte == one column in the terminal. Now it is assumed that
one multibyte character == one column. This works better with UTF-8.
Languages that only use single-width characters without any combining
characters should work correctly with this.

In xz, none of po/*.po contain combining characters and only ko.po,
zh_CN.po, and zh_TW.po contain fullwidth characters. Thus, "only"
those three translations in xz are broken on Windows with the
UTF-8 code page. Broken means that column headings in xz -lvv and
(only in the master branch) strings in --long-help are misaligned,
so it's not a huge problem. I don't know if those three languages
displayed perfectly before the UTF-8 change because I hadn't tested
translations with native Windows builds before.

Fixes: 46ee006162

src/common/tuklib_mbstr_width.c

@@ -12,7 +12,7 @@
 #include "tuklib_mbstr.h"
 #include <string.h>
 
-#if defined(HAVE_MBRTOWC) && defined(HAVE_WCWIDTH)
+#ifdef HAVE_MBRTOWC
 #	include <wchar.h>
 #endif
 
@@ -24,7 +24,7 @@ tuklib_mbstr_width(const char *str, size_t *bytes)
 	if (bytes != NULL)
 		*bytes = len;
 
-#if !(defined(HAVE_MBRTOWC) && defined(HAVE_WCWIDTH))
+#ifndef HAVE_MBRTOWC
 	// In single-byte mode, the width of the string is the same
 	// as its length.
 	return len;
@@ -46,11 +46,20 @@ tuklib_mbstr_width(const char *str, size_t *bytes)
 
 		i += ret;
 
+#ifdef HAVE_WCWIDTH
 		const int wc_width = wcwidth(wc);
 		if (wc_width < 0)
 			return (size_t)-1;
 
 		width += (size_t)wc_width;
+#else
+		// Without wcwidth() (like in a native Windows build),
+		// assume that one multibyte char == one column. With
+		// UTF-8, this is less bad than one byte == one column.
+		// This way quite a few languages will be handled correctly
+		// in practice; CJK chars will be very wrong though.
+		++width;
+#endif
 	}
 
 	// Require that the string ends in the initial shift state.