meson.build: Use -Wno-undef only for SDL2 versions that need it

meson.build: Use -Wno-undef only for SDL2 versions that need it

There is no need to disable this useful compiler warning for
all versions of the SDL. Unfortunately, various versions are
buggy (beside SDL 2.0.8, the version 2.26.0 and 2.26.1 are
broken, too, see https://github.com/libsdl-org/SDL/issues/6619 ),
but we can use a simple compiler check to see whether we need
the -Wno-undef or not.

This also enables the printing of the version number with
good versions of the SDL in the summary of the meson output
again.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230605114523.282987-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>