Fix gcc2 build on Mac OS X Lion.

The gcc2 cross-compiler built on Mac OS X Lion has a bug in it
where it is erroring with 'cast specifies signature type' when
assigning 0 or NULL to a pointer to a member fuction. NULL in this
instance is correctly converted to 0 since it is illegal to assign
((void*)0) to a pointer to a member function. However, it should
be legal to assign 0 to a pointer to a member function. Thus, there
is a bug.  Since I can't fix the gcc2 compiler I am working around
this bug by assigning the pointer to a do nothing function instead.

My host compiler version is
i686-apple-darwin11-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5666) (dot 3)

The same error occurs using the default gcc-llvm compiler and
a standard gcc 4.61 built from source. This bug does not occur on
Mac OS X 10.6 gcc2 or gcc4, nor does it occur on Mac OS X 10.7 with
the gcc4 cross-compiler.

If and when we decide to finally leave gcc2 behind we can revert this
change.

headers/libs/print/libprint/Halftone.h

@@ -102,6 +102,14 @@ protected:
 			void		SetupErrorBuffer(int x, int y, int width);
 			void		DitherFloydSteinberg(uchar* destination,
 							const uchar* source, int x, int y, int width);
+						// Do nothing method to get around a bug in
+						// the gcc2 cross-compiler built on Mac OS X
+						// Lion where a compiler error occurs when
+						// assigning a member function pointer to NULL
+						// or 0: cast specifies signature type.
+						// However, this should be legal according to
+						// the C++03 standard.
+			void		DitherNone(uchar*, const uchar*, int, int, int) {};
 
 private:
 	enum {