Cleanup: Convert winfunc and utfconv to C++

Basically, the intern/utfconv directory, as well as users of
these headers.

Pull Request: https://projects.blender.org/blender/blender/pulls/113901

extern/wcwidth/wcwidth.h

@@ -30,9 +30,17 @@ typedef unsigned int char32_t;
 #  endif
 #endif
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 int mk_wcwidth(char32_t ucs);
 int mk_wcswidth(const char32_t *pwcs, size_t n);
 int mk_wcwidth_cjk(char32_t ucs);
 int mk_wcswidth_cjk(const char32_t *pwcs, size_t n);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif