Add "project.extension" utility for declaring standalone extension projects.

Add "conditional" project utility for simplifying conditional requirements.


[SVN r34890]

tools/build

@@ -1 +1 @@
-Subproject commit 28d52b1e89060cb5d5468ba54da40846b72dc283
+Subproject commit dd8d2f36561601a2efa07bf2178a5cccc7f043e1