Fix recognizing locales using UTF-8 charset

Do not assume that C locale uses UTF-8, as this is simply not true and
none of the CRT functions handle UTF-8 correctly with this locale.

Do recognize locales explicitly using UTF-8 charset as being in UTF-8.
On most Unix systems (including Linux), it didn't really matter that we
didn't do it, because we used nl_langinfo() there, but it does matter a
lot with MSVC under MSW whose CRT supports UTF-8 now, but UTF-8
functions were not used there -- do use them now.

src/common/wxcrt.cpp

@@ -1041,6 +1041,19 @@ char *strdup(const char *s)
 bool wxLocaleIsUtf8 = false; // the safer setting if not known
 #endif
 
+static bool wxIsCharsetUtf8(const char* charset)
+{
+    if ( strcmp(charset, "UTF-8") == 0 ||
+         strcmp(charset, "utf-8") == 0 ||
+         strcmp(charset, "UTF8") == 0 ||
+         strcmp(charset, "utf8") == 0 )
+    {
+        return true;
+    }
+
+    return false;
+}
+
 static bool wxIsLocaleUtf8()
 {
     // NB: we intentionally don't use wxLocale::GetSystemEncodingName(),
@@ -1051,31 +1064,21 @@ static bool wxIsLocaleUtf8()
     // GNU libc provides current character set this way (this conforms to
     // Unix98)
     const char *charset = nl_langinfo(CODESET);
-    if ( charset )
-    {
-        // "UTF-8" is used by modern glibc versions, but test other variants
-        // as well, just in case:
-        if ( strcmp(charset, "UTF-8") == 0 ||
-             strcmp(charset, "utf-8") == 0 ||
-             strcmp(charset, "UTF8") == 0 ||
-             strcmp(charset, "utf8") == 0 )
-        {
-            return true;
-        }
-    }
-#endif // HAVE_LANGINFO_H
-
-    // check if we're running under the "C" locale: it is 7bit subset
-    // of UTF-8, so it can be safely used with the UTF-8 build:
+    if ( charset && wxIsCharsetUtf8(charset) )
+        return true;
+#else // !HAVE_LANGINFO_H
+    // check charset of the LC_CTYPE string: this also works with (sufficiently
+    // recent) MSVC and on any other system without nl_langinfo()
     const char *lc_ctype = setlocale(LC_CTYPE, nullptr);
-    if ( lc_ctype &&
-         (strcmp(lc_ctype, "C") == 0 || strcmp(lc_ctype, "POSIX") == 0) )
+    if ( lc_ctype )
     {
-        return true;
+        const char* charset = strrchr(lc_ctype, '.');
+        if ( charset && wxIsCharsetUtf8(charset + 1) )
+            return true;
     }
+#endif // HAVE_LANGINFO_H/!HAVE_LANGINFO_H
 
-    // we don't know what charset libc is using, so assume the worst
-    // to be safe:
+    // by default assume that we don't use UTF-8
     return false;
 }