x11/lxpanel and clang

[Kenta Suzumoto]

Forwarding to ports@ since kmoore just stopped maintaining the port.

Hi. lxpanel doesn't build with clang on my 9.1-release i386 system. I saw this PR about it: https://www.freebsd.org/cgi/query-pr.cgi?pr=176650 but it still doesn't build with clang.
Switching back to GCC lets it build, however. It should either be marked as requiring GCC or a real clang fix should be added (which I don't have at the moment).