Fix configure's libpng handling for cross compiling

By Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
	The build failures of bandwidthd observed in the autobuilders in
	relation to libpng are caused by the usage of -L/usr/lib
	-L/usr/local/lib in the compiler flags, which leads the configure
	script to try to link against a host libpng library.