Error at compiling rubygem rmagick.c

If the following error occurs:

compiling rmagick.c
In file included from rmagick.c:13:0:
rmagick.h:1210:51: error: unknown type name ‘MagickPixelPacket’; did you mean ‘PixelPacket’?

The reason is that rmagick is not compatible with ImageMagick 7. You have to install version 6.