[Yhc] Build(bot')ing Yhc with changing ghc versions

[Thorkil Naur]

Hello,

I have been changing my default ghc around lately on the machinery that I use 
for Yhc buildbotting. The original was ghc-6.4.1, but with a recent ghc-HEAD, 
I get errors like these:

  http://www.indiegigs.co.uk:8010/macosx_ppc_3/builds/15/step-yhc/0

...
scons: Building targets ...
Getting dependencies for src/libraries/core/Yhc/Core/Simplify.hs.
"ghc" -i./src/compiler98 -i./src/libraries/general -i./src/libraries/core 
-idepends/cpphs -idepends/filepath -i./src/interactive -package mtl 
-fwarn-missing-signatures -c src/libraries/core/Yhc/Core/Simplify.hs -o 
src/libraries/core/Yhc/Core/Simplify.o

src/libraries/core/Yhc/Core/Simplify.hs:7:0:
    Bad interface file: ./src/libraries/core/Yhc/Core/Type.hi
	mismatched interface file versions: expected 590528065, found 6
scons: *** [src/libraries/core/Yhc/Core/Simplify.o] Error 1
scons: building terminated because of errors.
program finished with exit code 2

I presume that this is because the buildbot-controlled build is incremental so 
that when I change the ghc version used, inconsistencies result.

So the question is this: When I change my ghc version, how do I clean up the 
Yhc buildbot build tree to ensure that everything gets built with the new ghc 
version?

Thanks and best regards
Thorkil