'file in wrong format' means there's different architectures involved. You're compiling as 64-bit and its trying to use your 32-bit gtk.
this fix should work:
I have problems compiling with my unofficial and unsupported multilib setup.
If you are using multilib packages from somewhere and also have some 32bit packages installed, then you might run into problems with the build process trying to link 32bit libraries instead of 64bit libraries. If this occurs, it can probably be solved by placing the following line:
after the lines for CFLAGS and CXXFLAGS passed to the configure script. In other words, make it look something like this:
Beyond that, you are on your own. This is an unsupported setup.
(instructions are from the SlackBuilds.org FAQ