Macでmysqlclientのpip install時に出るエラー

## 概要
Macで環境作り直して`pip install mysqlclient`しようとするとたまに出るエラー
いつも対処法を忘れるのでメモ

## エラー内容

```bash:エラー出力
    Running setup.py install for mysqlclient ... error
    ERROR: Command errored out with exit status 1:
     command: /Users/user/develop/api/venv/bin/python -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/z_/62bk1p8s7mj1p5_nq48s81gr0000gp/T/pip-install-8k8ewh_h/mysqlclient_980254ec895e421ba41a7ec66c0ccde2/setup.py'"'"'; __file__='"'"'/private/var/folders/z_/62bk1p8s7mj1p5_nq48s81gr0000gp/T/pip-install-8k8ewh_h/mysqlclient_980254ec895e421ba41a7ec66c0ccde2/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /private/var/folders/z_/62bk1p8s7mj1p5_nq48s81gr0000gp/T/pip-record-wtb2wvgb/install-record.txt --single-version-externally-managed --compile --install-headers /Users/user/develop/api/venv/include/site/python3.8/mysqlclient
         cwd: /private/var/folders/z_/62bk1p8s7mj1p5_nq48s81gr0000gp/T/pip-install-8k8ewh_h/mysqlclient_980254ec895e421ba41a7ec66c0ccde2/
    Complete output (70 lines):
    running install
    running build
    running build_py
    creating build
    creating build/lib.macosx-10.15-x86_64-3.8
    copying _mysql_exceptions.py -> build/lib.macosx-10.15-x86_64-3.8
    creating build/lib.macosx-10.15-x86_64-3.8/MySQLdb
    copying MySQLdb/__init__.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb
    copying MySQLdb/compat.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb
    copying MySQLdb/connections.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb
    copying MySQLdb/converters.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb
    copying MySQLdb/cursors.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb
    copying MySQLdb/release.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb
    copying MySQLdb/times.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb
    creating build/lib.macosx-10.15-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/__init__.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/CLIENT.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/CR.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/ER.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/FIELD_TYPE.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/FLAG.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb/constants
    copying MySQLdb/constants/REFRESH.py -> build/lib.macosx-10.15-x86_64-3.8/MySQLdb/constants
    running build_ext
    building '_mysql' extension
    creating build/temp.macosx-10.15-x86_64-3.8
    clang -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -Dversion_info=(1,3,14,'final',0) -D__version__=1.3.14 -I/usr/local/opt/mysql@5.7/include/mysql -I/Users/user/develop/api/venv/include -I/Users/user/.pyenv/versions/3.8.0/include/python3.8 -c _mysql.c -o build/temp.macosx-10.15-x86_64-3.8/_mysql.o
    _mysql.c:265:6: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                            s = PyUnicode_AsUTF8(item);
                              ^ ~~~~~~~~~~~~~~~~~~~~~~
    _mysql.c:295:6: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                            s = PyUnicode_AsUTF8(item);
                              ^ ~~~~~~~~~~~~~~~~~~~~~~
    _mysql.c:569:3: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                    _stringsuck(ca, value, ssl);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    _mysql.c:559:16: note: expanded from macro '_stringsuck'
            if(t){d=PyUnicode_AsUTF8(t);ssl_keepref[n_ssl_keepref++]=t;}\
                   ^~~~~~~~~~~~~~~~~~~~
    _mysql.c:570:3: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                    _stringsuck(capath, value, ssl);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    _mysql.c:559:16: note: expanded from macro '_stringsuck'
            if(t){d=PyUnicode_AsUTF8(t);ssl_keepref[n_ssl_keepref++]=t;}\
                   ^~~~~~~~~~~~~~~~~~~~
    _mysql.c:571:3: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                    _stringsuck(cert, value, ssl);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    _mysql.c:559:16: note: expanded from macro '_stringsuck'
            if(t){d=PyUnicode_AsUTF8(t);ssl_keepref[n_ssl_keepref++]=t;}\
                   ^~~~~~~~~~~~~~~~~~~~
    _mysql.c:572:3: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                    _stringsuck(key, value, ssl);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
    _mysql.c:559:16: note: expanded from macro '_stringsuck'
            if(t){d=PyUnicode_AsUTF8(t);ssl_keepref[n_ssl_keepref++]=t;}\
                   ^~~~~~~~~~~~~~~~~~~~
    _mysql.c:573:3: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                    _stringsuck(cipher, value, ssl);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    _mysql.c:559:16: note: expanded from macro '_stringsuck'
            if(t){d=PyUnicode_AsUTF8(t);ssl_keepref[n_ssl_keepref++]=t;}\
                   ^~~~~~~~~~~~~~~~~~~~
    _mysql.c:2557:8: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
            cname = PyUnicode_AsUTF8(name);
                  ^ ~~~~~~~~~~~~~~~~~~~~~~
    8 warnings generated.
    clang -bundle -undefined dynamic_lookup -L/usr/local/opt/readline/lib -L/usr/local/opt/readline/lib -L/Users/user/.pyenv/versions/3.8.0/lib -L/usr/local/opt/readline/lib -L/usr/local/opt/readline/lib -L/Users/user/.pyenv/versions/3.8.0/lib build/temp.macosx-10.15-x86_64-3.8/_mysql.o -L/usr/local/opt/mysql@5.7/lib -lmysqlclient -lssl -lcrypto -o build/lib.macosx-10.15-x86_64-3.8/_mysql.cpython-38-darwin.so
    ld: library not found for -lssl
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    error: command 'clang' failed with exit status 1
    ----------------------------------------

```

## 対応
環境変数`LDFFAGS`と`CPPFLAGS`を設定する

```bash:コマンド
export LDFLAGS="-L/usr/local/opt/openssl/lib"
export CPPFLAGS="-I/usr/local/opt/openssl/include"
pip install mysqlclient
```

## 参考
https://teratail.com/questions/214631