Our Clang --enable-pch setup is known broken

[Stephan Bergmann]

On 2/13/24 16:39, Stephan Bergmann wrote:
> Maybe somebody has the inclination to dig into our artisanal approach 
> and fix it (I assume that would involve fixing the underlying Clang 
> machinery that we tap into there), or rework gbuild to use Clang's 
> documented PCH approach.  Otherwise, I'd suggest to stick to the tried 
> and tested approach of not trying to use PCH in the first place...

or <https://gerrit.libreoffice.org/c/core/+/163422> "Work around some 
Clang PCH consteval issue by disabling HAVE_CPP_CONSTEVAL" to the 
rescue, for now