make compile-core: fix warnings

It is easier to spot real problems if the code is warning-free.

Replacing `gensym` with `make-symbol` is an idea taken from here:
b44c08dd45

In defer-until!:
core-lib.el:150:19:Warning: function ‘gensym’ from cl package called at
    runtime

In add-transient-hook!:
core-lib.el:216:16:Warning: function ‘gensym’ from cl package called at
    runtime

In toplevel form:
autoload/message.el:35:1:Warning: Unused lexical variable ‘spec’

In toplevel form:
autoload/line-numbers.el:31:1:Warning: defcustom for
    ‘display-line-numbers-type’ fails to specify containing group
autoload/line-numbers.el:31:1:Warning: defcustom for
    ‘display-line-numbers-type’ fails to specify containing group
autoload/line-numbers.el:39:1:Warning: defcustom for
    ‘display-line-numbers-grow-only’ fails to specify containing group
autoload/line-numbers.el:39:1:Warning: defcustom for
    ‘display-line-numbers-grow-only’ fails to specify containing group
autoload/line-numbers.el:44:1:Warning: defcustom for
    ‘display-line-numbers-width-start’ fails to specify containing group
autoload/line-numbers.el:44:1:Warning: defcustom for
    ‘display-line-numbers-width-start’ fails to specify containing group

In toplevel form:
cli/autoloads.el:137:1:Warning: Unused lexical variable ‘type’

Preserve name of unused lexical var _type

Makes it obvious what is stored there.

core/autoload/line-numbers.el

@@ -27,6 +27,9 @@ Lisp programs can disable display of a line number of a particular
 buffer line by putting the display-line-numbers-disable text property
 or overlay property on the first visible character of that line.")
 
+(defgroup display-line-numbers nil "Display line number preferences"
+ :group 'emacs)
+
 ;;;###autoload
 (defcustom display-line-numbers-type t
   "The default type of line numbers to use in `display-line-numbers-mode'.

core/autoload/message.el

@@ -47,7 +47,7 @@ Otherwise, it maps colors to a term-color-* face."
       (propertize
        text 'face
        (append (get-text-property 0 'face text)
-               (let (spec)
+               (let (_)
                  (cond ((>= code 40)
                         `(:background ,(caddr (assq style doom-ansi-alist))))
                        ((>= code 30)

core/cli/autoloads.el

@@ -192,7 +192,7 @@ even if it doesn't need reloading!"
                         (push `(put ',name 'doom-module ',origin) forms))))
 
                    ((eq type 'defalias)
-                    (cl-destructuring-bind (type name target &optional docstring) sexp
+                    (cl-destructuring-bind (_type name target &optional docstring) sexp
                       (let ((name (doom-unquote name))
                             (target (doom-unquote target)))
                         (unless (and member-p

core/core-lib.el

@@ -130,7 +130,7 @@ serve as a predicated alternative to `after!'."
   (declare (indent defun) (debug t))
   `(if ,condition
        (progn ,@body)
-     ,(let ((fun (gensym "doom|delay-form-")))
+     ,(let ((fun (make-symbol "doom|delay-form-")))
         `(progn
            (fset ',fun (lambda (&rest args)
                          (when ,(or condition t)
@@ -196,7 +196,7 @@ advised)."
   (let ((append (if (eq (car forms) :after) (pop forms)))
         (fn (if (symbolp (car forms))
                 (intern (format "doom|transient-hook-%s" (pop forms)))
-              (gensym "doom|transient-hook-"))))
+              (make-symbol "doom|transient-hook-"))))
     `(progn
        (fset ',fn
              (lambda (&rest _)