[LyX/master] Avoid bogus warnings when configuring for Qt6

[Enrico Forestieri]

On Tue, Jun 18, 2024 at 09:29:58PM +0100, José Matos wrote:
> 
>On Tue, 2024-06-18 at 20:04 +0000, Enrico Forestieri wrote:
>>     Avoid bogus warnings when configuring for Qt6
>
>Thank you. It works. :-)

Riki, Ok for stable?

-- 
Enrico