SOLVED: Plug-in build path missing standard entries

The situation:
When 9.5.0.2 came out, I upgraded that day (from .0.0) A while later,
and I decided I wanted to tweak my working Lpex extension plug-in. I
switched to the Plug-In Development Perspective, and my project had a
big red X on it. The first error was that com.ibm.lpex.core.LpexAction
could not be found. Which seemed weird, since that comes with RDi. And
in fact, the .JAR file is exactly where I expected to find it, so
somehow the 'internal CLASSPATH' is messed up.

More poking revealed that the Build Path for the project was completely
devoid of all of the .JAR files that are typically in there. All of
them being the core of the shipped product. Right click the project >
Properties > Java Build Path > Plug-in Dependencies. Expand that and
there should be 20-some .JAR files listed there, including
org.eclipse.ui_3.106.1.v20141002-1150.jar All were gone. Weird.

This really seemed like it was an install issue, not an issue with my
project; after all, the project had been working fine for quite some
time, and the only thing that had changed was the upgrade to 9.5 and
then to .0.2. I Googled to try to figure out where this setting was
kept, but came up empty. I verified that it was the entire install by
creating a new HelloWorld project, and it too showed the same class of
errors.


The solution:
I discovered that Help > Preferences > Plug-in Development > Target
Platform was set to 'Running Platform (Active)' at home, and was empty
on the non-working install at work. So I clicked 'Restore Defaults' and
voilà! With the Target Platform populated, the missing system .JAR
files are now included in the Build Path, and my project once again
compiles.

I don't know what messed it up, but at least the archives will show how
to set it straight.