Don't unnecessarily use non-ASCII characters in C sources

* src/xwidget.c (webkit_javascript_finished_cb):
* src/gtkutil.c (xg_check_special_colors):
* src/emacs-module.c (module_make_string):
* src/alloc.c (SET_STACK_TOP_ADDRESS): Don't use non-ASCII quote
characters.

src/alloc.c

@@ -5124,7 +5124,7 @@ typedef union
 #endif
 
 /* Set *P to the address of the top of the stack.  This must be a
-   macro, not a function, so that it is executed in the caller’s
+   macro, not a function, so that it is executed in the caller's
    environment.  It is not inside a do-while so that its storage
    survives the macro.  Callers should be declared NO_INLINE.  */
 #ifdef HAVE___BUILTIN_UNWIND_INIT

src/emacs-module.c

@@ -574,7 +574,7 @@ module_make_string (emacs_env *env, const char *str, ptrdiff_t length)
   if (! (0 <= length && length <= STRING_BYTES_BOUND))
     xsignal0 (Qoverflow_error);
   /* FIXME: AUTO_STRING_WITH_LEN requires STR to be null-terminated,
-     but we shouldn’t require that.  */
+     but we shouldn't require that.  */
   AUTO_STRING_WITH_LEN (lstr, str, length);
   return lisp_to_value (env,
                         code_convert_string_norecord (lstr, Qutf_8, false));

src/gtkutil.c

@@ -580,8 +580,8 @@ xg_check_special_colors (struct frame *f,
       {
         GdkRGBA *c;
         /* FIXME: Retrieving the background color is deprecated in
-           GTK+ 3.16.  New versions of GTK+ don’t use the concept of a
-           single background color any more, so we shouldn’t query for
+           GTK+ 3.16.  New versions of GTK+ don't use the concept of a
+           single background color any more, so we shouldn't query for
            it.  */
         gtk_style_context_get (gsty, state,
                                GTK_STYLE_PROPERTY_BACKGROUND_COLOR, &c,

src/xwidget.c

@@ -389,7 +389,7 @@ webkit_javascript_finished_cb (GObject      *webview,
     /* Register an xwidget event here, which then runs the callback.
        This ensures that the callback runs in sync with the Emacs
        event loop.  */
-    /* FIXME: This might lead to disaster if LISP_CALLBACK’s object
+    /* FIXME: This might lead to disaster if LISP_CALLBACK's object
        was garbage collected before now.  See the FIXME in
        Fxwidget_webkit_execute_script.  */
     store_xwidget_js_callback_event (xw, XIL ((intptr_t) lisp_callback),