Warn about Struct layout redefinition only

Since raising an exception is an API change but it doesn't justify a major version bump, just warn in that case.

lib/ffi/struct.rb

@@ -204,7 +204,7 @@ class << self
       #    end
       #  @note Creating a layout from a hash +spec+ is supported only for Ruby 1.9.
       def layout(*spec)
-        raise RuntimeError, "struct layout already defined for #{self.inspect}" if defined?(@layout)
+        warn "[DEPRECATION] Struct layout is already defined for class #{self.inspect}. Redefinition as in #{caller[0]} will be disallowed in ffi-2.0." if defined?(@layout)
         return @layout if spec.size == 0
 
         builder = StructLayoutBuilder.new

spec/ffi/struct_spec.rb

@@ -463,13 +463,24 @@ class TestStruct < FFI::Struct
       expect(s.offset_of(:c)).to eq(8)
     end
 
-    it "denies redefinition of struct layouts" do
-      expect do
-        Class.new(FFI::Struct) do
-          layout :a, :int
-          layout :a, :int
-        end
-      end.to raise_error(/struct layout already defined/)
+    if FFI::VERSION < "2"
+      it "warns about redefinition of struct layouts" do
+        expect do
+          Class.new(FFI::Struct) do
+            layout :a, :int
+            layout :a, :int
+          end
+        end.to output(/Redefinition .* will be disallowed in ffi-2.0/).to_stderr
+      end
+    else
+      it "denies redefinition of struct layouts" do
+        expect do
+          Class.new(FFI::Struct) do
+            layout :a, :int
+            layout :a, :int
+          end
+        end.to raise_error(/struct layout already defined/)
+      end
     end
 
     it "allows redefinition of struct layouts in derived classes" do