sconeserver: fix imagemagick build failures

sconeserver: fix imagemagick build failures

The sconeserver configure script uses Magick++-config to obtain compiler and
linker arguments for the imagemagick libraries. This doesn't work in
Buildroot, and causes build failures such as the following

  http://autobuild.buildroot.net/results/7833edd60bbb2c291aea1efb5ccf407da7f4a51f/build-end.log

The upstream sconeserver code has been changed to use pkg-config directly,
instead of Magick++-config; this patch adapts the sconeserver package to
the upstream change.

Both this patch and the associated upstream change are based on a Buildroot
patch submitted by Samuel Martin: http://patchwork.ozlabs.org/patch/200901/

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>