[cairo] cairo tests depend on PNG functions

[Jason Dorje Short]

If libpng isn't available then the cairo PNG functions won't be 
available and the cairo tests won't be able to compile - a bug.

This patch simply disables the test program if the PNG functions aren't 
available.  Note the test program doesn't (yet) use the cairo PNG 
functions; it uses libpng directly (read-png.c and write-png.c).  But 
the condition for success is the same (libpng being present).

-jason
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