It's actually quite easy to get it to compile, but it took me a while to figure out what the issue was at first. Here's what you have to do.

1. Install gmake, gcc, and git using pkg_add if you don't already have them. You may need to install xcb as well.
2. Clone the lemonbar repo.
3. Open the Makefile in your editor of choice.
4. Change gcc to egcc on the line where it says CC.
5. Add -I /usr/X11R6/include to the end of the line where it says CFLAGS.
6. Add -L /usr/X11R6/lib to the end of the line where it says LDFLAGS.
7. Under where it says install, change the paths so that the lemonbar executable goes in /usr/local/bin, and the man page goes in /usr/local/man.
8. Save and close the Makefile.
9. Run gmake install.

Hope this helps someone.