Raise an error when the specified OpenSSL library directory doesn't exist.

OpenSSL creates the library directory to the `/path/to/openssl_dir/lib64`
as a default, when it is built from the source as follow.
In the case, the `bundle exec rake compile -- --with-openssl-dir=<openssl_dir>`
cannot compile with the lib64 directory, and may compile with system OpenSSL's
libraries unintentionally. This commit is a check to prevent the case.

Here is the example.

```
$ ls -l /path/to/openssl_dir/
...
drwxr-xr-x. 5 jaruga jaruga 4096 Jul 17 23:06 lib64/
...

$ bundle exec rake compile -- \
  --with-openssl-dir=/path/to/openssl_dir
...
../../../../ext/openssl/extconf.rb:21:in `<main>': OpenSSL library directory could not be found in '/path/to/openssl_dir/lib'. You might want to use --with-openssl-lib=<dir> option to specify the directory. (RuntimeError)
rake aborted!
...
```

To avoid this error, you can compile with the command below.

```
$ bundle exec rake compile -- \
  --with-openssl-dir=/path/to/openssl \
  --with-openssl-lib=/path/to/openssl/lib64
```

Author: junaruga

ext/openssl/extconf.rb

@@ -13,7 +13,15 @@
 
 require "mkmf"
 
-dir_config_given = dir_config("openssl").any?
+openssl_dirs = dir_config("openssl")
+dir_config_given = openssl_dirs.any?
+_, ldir = openssl_dirs
+if ldir&.split(File::PATH_SEPARATOR)&.none? { |dir| File.directory?(dir) }
+  raise "OpenSSL library directory could not be found in '#{ldir}'. " \
+    'You might want to use --with-openssl-lib=<dir> option to specify the ' \
+    'directory.'
+end
+
 dir_config("kerberos")
 
 Logging::message "=== OpenSSL for Ruby configurator ===\n"