Re: [PATCH] cmake/build.mk: Hide non-buildlink3 libraries from cmake

[Thomas Klausner <wiz%NetBSD.org@localhost>]

With the patch, I see weird problems in some KDE packages like
kguiaddons (but also others):

In file included from /scratch/x11/kguiaddons/work/.buildlink/include/wayland-client-core.h:30,
                 from /scratch/x11/kguiaddons/work/.buildlink/include/wayland-client.h:39,
                 from /scratch/x11/kguiaddons/work/kguiaddons-5.116.0/_KDE_build/src/wayland-keystate-client-protocol.h:8,
                 from /scratch/x11/kguiaddons/work/kguiaddons-5.116.0/_KDE_build/src/qwayland-keystate.h:7,
                 from /scratch/x11/kguiaddons/work/kguiaddons-5.116.0/_KDE_build/src/qwayland-keystate.cpp:4:
/scratch/x11/kguiaddons/work/.buildlink/include/wayland-util.h:51:5: error: "__STDC_VERSION__" is not defined, evaluates to 0 [-Werror=undef]
   51 | #if __STDC_VERSION__ >= 202311L
      |     ^~~~~~~~~~~~~~~~
/scratch/x11/kguiaddons/work/.buildlink/include/wayland-util.h:73:5: error: "__STDC_VERSION__" is not defined, evaluates to 0 [-Werror=undef]
   73 | #if __STDC_VERSION__ >= 202311L
      |     ^~~~~~~~~~~~~~~~

__STDC_VERSION__ is only defined when compiling C code, but the header
is included from a C++ source file and thus we get this warning.

But why don't we get it without the cmake changes?
 Thomas