JIT

Add warning when building with custom LLVM_VERSION #21

Job	Run time
Interpreter (Debug)	9m 45s
x86_64-pc-windows-msvc/msvc (Release)	15m 56s
i686-pc-windows-msvc/msvc (Release)	14m 19s
i686-pc-windows-msvc/msvc (Debug)	23m 51s
aarch64-apple-darwin/clang (Release)	9m 34s
aarch64-pc-windows-msvc/msvc (Release)	18m 8s
aarch64-pc-windows-msvc/msvc (Debug)	23m 3s
aarch64-unknown-linux-gnu/gcc (Debug)	6m 48s
x86_64-unknown-linux-gnu/gcc (Debug)	11m 43s
x86_64-apple-darwin/clang (Debug)	19m 9s
x86_64-pc-windows-msvc/msvc (Debug)	23m 25s
aarch64-unknown-linux-gnu/gcc (Release)	5m 49s
x86_64-unknown-linux-gnu/gcc (Release)	7m 42s
x86_64-apple-darwin/clang (Release)	14m 38s
aarch64-apple-darwin/clang (Debug)	10m 23s
	3h 34m 13s

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