config(8): fixes for -fno-common

Move this handful of definitions into main.c, properly declare these as
extern in config.h. This fixes the config(8) build with -fno-common.

Unexplained in my previous commit to gas, -fno-common will become the
default in GCC10 and LLVM11, so it's worth addressing these in advance.

MFC after: 3 days