Looks like I get to chalk this one up to user error. The new project I built worked fine, so I started looking at the .cbp files to compare differences. I found a pre-build command that specifically calls mingw32-make -- obviously the source of this error.
<ExtraCommands>
<Add before="mingw32-make -f esim.mak targetos=win32 hostos=win32 dbfiles" />
</ExtraCommands>
According to my svn, this line has been present in the project file since 13-Mar-2008 !!
If I right-click the project, select
Build Options->
Pre/Post Build Steps sure enough it's there. I didn't notice it before because it's attached to the project name and not one of the build configurations (ie when I looked before, I had the "debug" build configuration selected, in which case the pre-build steps is shown as empty)
In CB-8.02, the pre-build steps window is grayed out, I can't make any changes to the existing command (and the pre-build command that is displayed never actually runs!). In CB-5731, I can modify this line. Now I've changed it to what it should be and all is well again.
Is it known that CB-8.02 doesn't run pre-build commands? This would definitely explain one sporadic error I've witnessed here.