Without the change build fails as:
$ nix build --impure --expr 'with import ./. {}; xwinmosaic.overrideAttrs (oa: { NIX_CFLAGS_COMPILE = (oa.NIX_CFLAGS_COMPILE or "") + "-fno-common"; })'
ld: CMakeFiles/xwinmosaic.dir/main.c.o:(.bss+0xc0):
multiple definition of `a_NET_CURRENT_DESKTOP'; CMakeFiles/xwinmosaic.dir/x_interaction.c.o:(.bss+0xc0): first defined here
ld: CMakeFiles/xwinmosaic.dir/main.c.o:(.bss+0xc8):
multiple definition of `a_NET_DESKTOP_VIEWPORT'; CMakeFiles/xwinmosaic.dir/x_interaction.c.o:(.bss+0xc8): first defined here
Sergei Trofimovich