Cache: implement "dirty" attribute to only re-serialize when necessary

lib/rscons/cache.rb

@@ -75,10 +75,13 @@ module Rscons
 
     # Write the cache to disk to be loaded next time.
     def write
-      @cache["version"] = VERSION
-      File.open(CACHE_FILE, "w") do |fh|
-        fh.puts(JSON.dump(@cache))
+      if @dirty || (@cache["version"] != VERSION)
+        @cache["version"] = VERSION
+        File.open(CACHE_FILE, "w") do |fh|
+          fh.puts(JSON.dump(@cache))
+        end
       end
+      @dirty = false
     end
 
     # Check if target(s) are up to date
@@ -164,6 +167,7 @@ module Rscons
             }
           end,
         }
+        @dirty = true
       end
     end
 
@@ -181,6 +185,7 @@ module Rscons
         unless File.exists?(subpath)
           FileUtils.mkdir(subpath)
           @cache["directories"][subpath] = true
+          @dirty = true
         end
       end
     end
@@ -203,6 +208,7 @@ module Rscons
       @cache["targets"] ||= {}
       @cache["directories"] ||= {}
       @lookup_checksums = {}
+      @dirty = false
     end
 
     # Return a file's checksum, or the previously calculated checksum for