JIT

Move LLVM_VERSION env var detection to configure #18

Job	Run time
Interpreter (Debug)	9m 57s
i686-pc-windows-msvc/msvc (Release)	15m 48s
x86_64-pc-windows-msvc/msvc (Release)	14m 46s
x86_64-pc-windows-msvc/msvc (Debug)	23m 49s
aarch64-pc-windows-msvc/msvc (Release)	18m 33s
x86_64-apple-darwin/clang (Debug)	24m 21s
aarch64-pc-windows-msvc/msvc (Debug)	23m 17s
aarch64-unknown-linux-gnu/gcc (Debug)	7m 5s
x86_64-unknown-linux-gnu/gcc (Release)	7m 46s
aarch64-apple-darwin/clang (Release)	9m 20s
aarch64-apple-darwin/clang (Debug)	15m 8s
aarch64-unknown-linux-gnu/gcc (Release)	5m 41s
x86_64-apple-darwin/clang (Release)	18m 2s
x86_64-unknown-linux-gnu/gcc (Debug)	10m 59s
i686-pc-windows-msvc/msvc (Debug)	25m 15s
	3h 49m 47s

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