add skpdiff tool to compare bitmaps

Makefile

 # Makefile that wraps the Gyp and build steps for Unix and Mac (but not Windows)
 # Uses "make" to build on Unix, and "xcodebuild" to build on Mac.
 #
 # Some usage examples (tested on both Linux and Mac):
 #
 #   # Clean everything
 #   make clean
 #
 #   # Build and run tests (in Debug mode)
 #   make tests
@@ skipping 30 matching lines @@
                  debugger \
                  everything \
                  gm \
                  most \
                  pathops_unittest \
                  pdfviewer \
                  SampleApp \
                  SkiaAndroidApp \
                  skia_lib \
                  tests \
-                 tools
+                 tools \
+                 skpdiff
 
 # Default target.  This must be listed before all other targets.
 .PHONY: default
 default: most
 
 # As noted in http://code.google.com/p/skia/issues/detail?id=330 , building
 # multiple targets in parallel was failing.  The special .NOTPARALLEL target
 # tells gnu make not to run targets within _this_ Makefile in parallel, but the
 # recursively invoked Makefile within out/ _is_ allowed to run in parallel
 # (so you can still get some speedup that way).
@@ skipping 56 matching lines @@
 else ifneq (,$(findstring make, $(GYP_GENERATORS)))
         $(MAKE) -C $(SKIA_OUT) $@ BUILDTYPE=$(BUILDTYPE)
 else ifneq (,$(findstring Darwin, $(uname)))
         rm -f out/$(BUILDTYPE) || if test -d out/$(BUILDTYPE); then echo "run 'make clean' or otherwise delete out/$(BUILDTYPE)"; exit 1; fi
         xcodebuild -project out/gyp/$@.xcodeproj -configuration $(BUILDTYPE)
         ln -s $(CWD)/xcodebuild/$(BUILDTYPE) out/$(BUILDTYPE)
 else
         echo "unknown platform $(uname)"
         exit 1
 endif