You need to install gc which provides gc.h. This is noted here
This requires gc and imlib2
So check which packages you should build and install as dependencies and install it first. Unless otherwise stated these dependencies are available @ slackbuilds.org as well.
And remember that slackbuilds.org assume that you made a full install of Slackware thus if a needed dependency is included in a full install it won't be mentioned.
If after having installed all dependencies you still have a problem do what I suggested you in my previous answer.
To append the log (netcatlog.txt in the example I gave you) click "Go Advanced" whilst editing your post, then click on the paperclip in the icons menu, hit "browse" to select your file then hit "upload". This way you won't have to copy/paste anything.
PS May be you meant that the buttons I just mentioned are not usable through links. Can't you use another browser then?