Check a OpenSSL library directory existence.

Add a logic to check a OpenSSL library directory existence on the given
`--with-openssl-dir=<dir>` option.

OpenSSL creates the library directory to the `/path/to/openssl_dir/lib64`
as a default, when it is built from the source as follow.

OpenSSL
```
$ ./Configure \
  --prefix=/path/to/openssl
$ make
$ make install
```

In the case of the following command, the `dir_config("openssl")` returns
`["/path/to/openssl/include", "/path/to/openssl/lib"]`. And this causes the
Ruby OpenSSL binding is unintentionally built with the system OpenSSL's library
directory, because the `/path/to/openssl/lib` doesn't exist. The logic to check
the OpenSSL library directory's existence is to avoid building in this case.

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

In the case of the following command, the `dir_config("openssl")` returns
`["/path/to/openssl/include",
"/path/to/openssl/lib64:/path/to/openssl/lib64:/path/to/openssl/lib"]`.
The returned library directory string is a set of the directories with the path
separator ":".

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

ext/openssl/extconf.rb

@@ -13,7 +13,16 @@
 
 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"