numbaインストール時のLLVMエラーの原因と解決法

## 実行環境
Ubuntu:18.04
python:3.8.0
numba:0.55.1


## numbaをinstall時に下記のエラーが発生
```:実行コマンド
$ pip install numba
```
```:エラー全文
Collecting numba
  Using cached https://files.pythonhosted.org/packages/69/df/bd36068b2c1d0d34794f8ac0c222f9c4ad88dc710b400e65dbb3b59ea57e/numba-0.55.1.tar.gz
Collecting llvmlite<0.39,>=0.38.0rc1 (from numba)
  Using cached https://files.pythonhosted.org/packages/d8/e3/bd329a96549809598acd5daaccd35fd9d0883185cfe7f681a9e3e54beaa0/llvmlite-0.38.0.tar.gz
Collecting numpy<1.22,>=1.18 (from numba)
  Using cached https://files.pythonhosted.org/packages/d5/43/e88bb1fb7d040ae8e0e06e749341b13f57701aab11fe9d71c99af6202c5c/numpy-1.21.6-cp38-cp38-manylinux_2_12_x86_64.manylinux2010_x86_64.whl
Requirement already satisfied: setuptools in /home/$(user)/.pyenv/versions/3.8.0/envs/$(virtualenv name)/lib/python3.8/site-packages (from numba) (41.2.0)
Installing collected packages: llvmlite, numpy, numba
  Running setup.py install for llvmlite ... error
    ERROR: Command errored out with exit status 1:
     command: /home/$(user)/.pyenv/versions/3.8.0/envs/$(virtualenv name)/bin/python3.8 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-0e_cn8e1/llvmlite/setup.py'"'"'; __file__='"'"'/tmp/pip-install-0e_cn8e1/llvmlite/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-n9rj4frp/install-record.txt --single-version-externally-managed --compile --install-headers /home/$(user)/.pyenv/versions/3.8.0/envs/$(virtualenv name)/include/site/python3.8/llvmlite
         cwd: /tmp/pip-install-0e_cn8e1/llvmlite/
    Complete output (18 lines):
    running install
    running build
    got version from file /tmp/pip-install-0e_cn8e1/llvmlite/llvmlite/_version.py {'version': '0.38.0', 'full': 'f0365b91ce1e1f74c68785c6d0067f32f89857d9'}
    running build_ext
    /home/muratam39/.pyenv/versions/3.8.0/envs/call/bin/python3.8 /tmp/pip-install-0e_cn8e1/llvmlite/ffi/build.py
    LLVM version... 6.0.0
    
    Traceback (most recent call last):
      File "/tmp/pip-install-0e_cn8e1/llvmlite/ffi/build.py", line 220, in <module>
        main()
      File "/tmp/pip-install-0e_cn8e1/llvmlite/ffi/build.py", line 210, in main
        main_posix('linux', '.so')
      File "/tmp/pip-install-0e_cn8e1/llvmlite/ffi/build.py", line 172, in main_posix
        raise RuntimeError(msg)
    RuntimeError: Building llvmlite requires LLVM 11.x.x, got '6.0.0'. Be sure to set LLVM_CONFIG to the right executable path.
    Read the documentation at http://llvmlite.pydata.org/ for more information about building llvmlite.
    
    error: command '/home/$(user)/.pyenv/versions/3.8.0/envs/$(virtualenv name)/bin/python3.8' failed with exit status 1
    ----------------------------------------
ERROR: Command errored out with exit status 1: /home/$(user)/.pyenv/versions/3.8.0/envs/$(virtualenv name)/bin/python3.8 -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-0e_cn8e1/llvmlite/setup.py'"'"'; __file__='"'"'/tmp/pip-install-0e_cn8e1/llvmlite/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-n9rj4frp/install-record.txt --single-version-externally-managed --compile --install-headers /home/$(user)/.pyenv/versions/3.8.0/envs/$(virtualenv name)/include/site/python3.8/llvmlite Check the logs for full command output.
```

## 原因と解決策
```:原因箇所
RuntimeError: Building llvmlite requires LLVM 11.x.x, got '6.0.0'. Be sure to set LLVM_CONFIG to the right executable path.
```

llvmliteのbuild時に必要なLLVMライブラリのバージョンが違うことが原因。
下記のコマンドを実行して、LLVMライブラリのバージョンを必要なものと合わせてから再インストールすると解決する。
```
wget https://apt.llvm.org/llvm.sh
chmod +x llvm.sh
sudo ./llvm.sh <version number>(今回で言うと11)

export LLVM_CONFIG=/usr/bin/llvm-config-11  # パスを通す

pip install numba
```