gd: ensure libpng-config from staging is used

The gd package configure call 'libpng-config' to get the compiler
flags required to use the libpng. The configure correctly allow to
specify the path of the staging libpng-config by using the
ac_cv_path_LIBPNG_CONFIG but the configure.ac call simply
'libpng-config' instead of the specified one. The configure.ac is now
modified to call the specified libpng_config.

[Peter: explictly pass --without-png instead of auto detect]
Signed-off-by: Jean-Christian de Rivaz <jc@eclis.ch>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>