Simplify demo build process

[EvanKirshenbaum]

The current setup and build for the inventory demo is still pretty complicated, even with the ant build file. In particular,

• It doesn't automatically build createheap.
• The way to choose which build config to use is awkward.
• It often seems to rebuild things it doesn't need to (for C++) and doesn't notice needed changes (for Java).
• It requires a lot of manually-created symbolic links.

I'd like to change this to

• Do a make install-recursive in the build config of your choice inside of java-api.
• Do an ant build (or, perhaps, a make) in demo/inventory to compile only the demo Java code.
• Have the latter and the scripts expect to find things in the install hierarchy.