package/imagemagick: utilities now need C++ support

Since upstream commit
https://github.com/ImageMagick/ImageMagick/commit/07f3b487f9860fd4eb9422f1a906d0fe83b6fd1c
(which first appeared in version 7.1.0-47), ImageMagick forces the
need of a C++ compiler to build its utilities. Despite the request of
Bernd Kuhls to revert this change, upstream declined.

Since this change is causing build failures in our autobuilders, our
only choice is to follow the choice of upstream, and disable building
the utilities when C++ support is not available.

Fixes:

  http://autobuild.buildroot.net/results/4283235d697408cf2e70be5e3769dbe6ebb9ddae/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>