better gerber export handling

hardware/Rakefile

@@ -97,9 +97,15 @@ task :photo => photos
 CLOBBER.include(photos)
 
 desc "export gerber"
-task :gerber do
+task :gerber => :version do
   targets.each do |target|
-    sh "pcb -x gerber --gerberfile #{target[:name]} --all-layers #{target[:vpcb]}"
+    export = true # export only if the gerbers are all older than the layout
+    Dir.foreach(".") do |file|
+      next unless file.start_with? target[:name]
+      next unless file.end_with? ".gbr" or file.end_with? ".cnc"
+      export &= (File.ctime(target[:vpcb])>File.ctime(file))
+    end
+    sh "pcb -x gerber --gerberfile #{target[:name]} --all-layers #{target[:vpcb]}" if export
   end
 end
 CLOBBER.include("*.gbr")