net-mgmt/mk-livestatus: fix "ld: error: duplicate symbol: g_mainthread_id"

• add '-fcommon' to our CFLAGS in order to build with clang 11;
• fix used by other ports as well (mentioned a few on original PR);
• this is applied in a conditional way, checking OSVERSION;
• while here, also added new DISTSITES.

Reported by: dvl@, pkg-fallout
PR: 257162