[mkgmap-dev] Question on license for style-file

I am going to publish my style-file (and dual license the rest like typfiles), but I would like that a) any works that build upon it, have to give attribution to openmtbmap.org b) any maps generated by using the style-file or large parts of it have to give attribution to openmtbmap.org Which license does fit here. Is GnuGPL v2 compatible with my intention or is b) not possible? Would CCBYSA 3.0 be better? Hope someone knows a bit better what I should choose. I don't really understand how b) is treated by open-source licenses.