scons: Build progs.

Just demos and trivial dirs for starters.
diff --git a/progs/SConscript b/progs/SConscript
new file mode 100644
index 0000000..245fc26
--- /dev/null
+++ b/progs/SConscript
@@ -0,0 +1,4 @@
+SConscript([
+    'demos/SConscript',
+    'trivial/SConscript',
+])