ext/openssl/extconf.rb: check with -Werror=deprecated-declarations

This reverts commit 0d7d8b2989,
but restore `$warnflags` without the flag, to get rid of using
deprecated functions.

ext/openssl/deprecation.rb

@@ -1,7 +1,23 @@
 # frozen_string_literal: false
 module OpenSSL
+  def self.deprecated_warning_flag
+    unless flag = (@deprecated_warning_flag ||= nil)
+      if try_compile("", flag = "-Werror=deprecated-declarations")
+        $warnflags = "#{@warnflags = $warnflags}" #{flag}"
+      else
+        flag = ""
+      end
+      @deprecated_warning_flag = flag
+    end
+    flag
+  end
+
+  def self.restore_warning_flag
+    $warnflags = @warnflags
+  end
+
   def self.check_func(func, header)
-    have_func(func, header)
+    have_func(func, header, deprecated_warning_flag)
   end
 
   def self.check_func_or_macro(func, header)

ext/openssl/extconf.rb

@@ -19,6 +19,9 @@ dir_config("kerberos")
 
 Logging::message "=== OpenSSL for Ruby configurator ===\n"
 
+# Check with -Werror=deprecated-declarations if available
+OpenSSL.deprecated_warning_flag
+
 ##
 # Adds -DOSSL_DEBUG for compilation and some more targets when GCC is used
 # To turn it on, use: --with-debug or --enable-debug
@@ -166,5 +169,6 @@ have_func("EVP_PBE_scrypt")
 Logging::message "=== Checking done. ===\n"
 
 create_header
+OpenSSL.restore_warning_flag
 create_makefile("openssl")
 Logging::message "Done.\n"