nrpe: fix system openssl linkage

Closes #35024.
Yeh, providing you use the ` --with-mysql ` option:

```
checking for mysql_config... /usr/local/opt/mysql/bin/mysql_config
checking for mysql_init in -lmysqlclient... yes
--with-mysql: /usr/local/opt/mysql/bin/mysql_config
```

Otherwise:

```
checking for mysql_config... no
configure: WARNING: Skipping mysql plugin
configure: WARNING: install mysql client libs to compile this plugin (see REQUIREMENTS).
```