scons: Print a deprecation warning about using scons on not windows

At this point meson should be able to handle all of the non-windows
platforms just fine; we'd like to be able to stop maintaining scons for
those platforms sooner than later.

Reviewed-by: Eric Anholt <eric@anholt.net>

.gitlab-ci/scons-build.sh

@@ -8,5 +8,5 @@ if test -n "$LLVM_VERSION"; then
 fi
 
 rm -rf build
-scons $SCONS_TARGET
-eval $SCONS_CHECK_COMMAND
+scons $SCONS_TARGET force_scons=on
+eval $SCONS_CHECK_COMMAND