ext/openssl/extconf.rb: do not use -Werror=deprecated-declarations

This is a combined commit of the following commits by mame and nobu:

 - 0d7d8b2989e1 ("ext/openssl/extconf.rb: do not use -Werror=deprecated-declarations", 2019-12-05)
 - c3abbc1b2f52 ("ext/openssl/extconf.rb: check with -Werror=deprecated-declarations", 2019-12-05)

-Werror=deprecated-declarations should only be used while checking
available features, and not for compiling the extension itself.

This is a backport to the 2.1 branch from ruby.git. Note that current
master (targeting 3.0) completely removed ext/openssl/deprecation.rb.

Author: rhenium

ext/openssl/deprecation.rb

@@ -3,7 +3,7 @@ module OpenSSL
   def self.deprecated_warning_flag
     unless flag = (@deprecated_warning_flag ||= nil)
       if try_compile("", flag = "-Werror=deprecated-declarations")
-        $warnflags << " #{flag}"
+        $warnflags = "#{@warnflags = $warnflags}" #{flag}"
       else
         flag = ""
       end
@@ -12,6 +12,10 @@ def self.deprecated_warning_flag
     flag
   end
 
+  def self.restore_warning_flag
+    $warnflags = @warnflags
+  end
+
   def self.check_func(func, header)
     have_func(func, header, deprecated_warning_flag)
   end

ext/openssl/extconf.rb

@@ -19,7 +19,7 @@
 
 Logging::message "=== OpenSSL for Ruby configurator ===\n"
 
-# Add -Werror=deprecated-declarations to $warnflags if available
+# Check with -Werror=deprecated-declarations if available
 OpenSSL.deprecated_warning_flag
 
 ##
@@ -169,5 +169,6 @@ def find_openssl_library
 Logging::message "=== Checking done. ===\n"
 
 create_header
+OpenSSL.restore_warning_flag
 create_makefile("openssl")
 Logging::message "Done.\n"