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

OpenSSL built from the source creates the library directory to the
`/path/to/openssl_dir/lib64` as a default.
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 to check this case to avoid linking
with an unintentional library directory.

Author: junaruga

ext/openssl/extconf.rb

@@ -13,12 +13,21 @@
 
 require "mkmf"
 
+ssl_dirs = nil
 if defined?(::TruffleRuby)
   # Always respect the openssl prefix chosen by truffle/openssl-prefix
   require 'truffle/openssl-prefix'
-  dir_config_given = dir_config("openssl", ENV["OPENSSL_PREFIX"]).any?
+  ssl_dirs = dir_config("openssl", ENV["OPENSSL_PREFIX"])
 else
-  dir_config_given = dir_config("openssl").any?
+  ssl_dirs = dir_config("openssl")
+end
+dir_config_given = ssl_dirs.any?
+
+_, ssl_ldir = ssl_dirs
+if ssl_ldir&.split(File::PATH_SEPARATOR)&.none? { |dir| File.directory?(dir) }
+  msg = "OpenSSL library directory could not be found in '#{ssl_ldir}'. " \
+    'See <link> to fix this error.'
+  raise msg
 end
 
 dir_config("kerberos")