JIT

Move LLVM_VERSION env var detection to configure #17

Job	Run time
Interpreter (Debug)	9m 42s
x86_64-unknown-linux-gnu/gcc (Release)	6m 46s
i686-pc-windows-msvc/msvc (Debug)	20m 57s
i686-pc-windows-msvc/msvc (Release)	13m 0s
aarch64-pc-windows-msvc/msvc (Release)	16m 28s
x86_64-pc-windows-msvc/msvc (Debug)	22m 42s
aarch64-apple-darwin/clang (Release)	11m 28s
aarch64-pc-windows-msvc/msvc (Debug)	23m 23s
x86_64-apple-darwin/clang (Debug)	21m 39s
aarch64-unknown-linux-gnu/gcc (Debug)	7m 5s
aarch64-apple-darwin/clang (Debug)	12m 57s
x86_64-apple-darwin/clang (Release)	13m 0s
aarch64-unknown-linux-gnu/gcc (Release)	8m 21s
x86_64-pc-windows-msvc/msvc (Release)	11m 45s
x86_64-unknown-linux-gnu/gcc (Debug)	11m 40s
	3h 30m 53s

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