to fix issue Target llvm is not enabled

[henrywu2019]

Issue:
I was building TVM4J and got error bf != nullptr Target llvm is not enabled when generating shared library. The issue is similar to #1020

Solution:
enable LLVM in config.cmake

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[FrozenGene]

I think we shouldn't do this change. As the comment said:

/path/to/llvm-config: enable specific LLVM when multiple llvm-dev is available.

We should let the user specify the correct path. The doc also said what the condition and how to turn it on:
https://docs.tvm.ai/install/from_source.html Users should read the manual carefully firstly.

Meanwhile, when we compile the runtime only, we don't need LLVM on. So TVM just depend on LLVM optionally.

[henrywu2019]

I see. Maybe a better way is to put it explicitly in TVM4J's README saying we need to turn on LLVM support for TVM4J? It will help beginners to save a lot of time to troubleshooting, won't it?

[FrozenGene]

I see. Maybe a better way is to put it explicitly in TVM4J's README saying we need to turn on LLVM support for TVM4J? It will help beginners to save a lot of time to troubleshooting, won't it?

The TVM4J Readme in fact have listed the doc I mention before:

However, I think you could PR the TVM4J readme and emphasize users should notice turning LLVM on.