I had two issues:
1. I tried first to emerge just lxpanel 0.7.0-r1, and it refused to do that. I realized (by reading the messages from make) that one dependency wasn't properly installed, it was libfm-gtk. I decided to install the whole lxde-meta package, since pcmanfm depends on libfm-gtk too, and then it compiled successfully. I guess it should have been detected as a dependency at first.

2. Even after having compiled, lxpanel crashes when I use "lxpanelctl run", and just say "segmentation fault". I used this same version of lxpanel with my previous Arch Linux installation and it worked fine in this same pc, so I guess it's not a version issue.