Only test for bug #9439 when llvm is installed

When running ./configure to build GHC, and opt is not found, the current output is:

<no location info>: Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM ghc: could not execute: opt failed to compile

When I first saw this, I thought that this was causing the entire compiler build to fail. To clarify that this isn't a critical error, should it be something more like:

checking for llvm... no

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	DocumentationBug
Priority	low
Resolution	Unresolved
Component	Build System
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