gcc-10 changed the default from -fcommon to fno-common:
  https://gcc.gnu.org/PR85678

As a result build fails as:

    ld: src/log.o:(.bss+0x0): multiple definition of `W'; src/barwin.o:(.bss+0x0): first defined here
    ld: src/wmfs.o:(.bss+0x0): multiple definition of `W'; src/barwin.o:(.bss+0x0): first defined here
    ld: src/wmfs.o:(.bss+0x10): multiple definition of `event_handle'; src/client.o:(.bss+0x10): first defined here

The change moves definitions into .c files.