patch: robustify libpng check.

[Stefan Seefeld]

The attached patch robustifies the check for libpng by not only looking 
for the header, but also attempting to link. Previously we ran into a 
situation where libpng.h was found (as part of a 32-bit package, but the 
64-bit version of the library then was not, resulting in a failure at 
compile-time, instead of simply disabling png-support at configure time.

The patch is checked in as obvious.

Thanks,
		Stefan

-- 
Stefan Seefeld
CodeSourcery
stefan at codesourcery.com
(650) 331-3385 x718
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