yeahwm: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

    ld: screen.o:(.bss+0x40): multiple definition of `fg'; client.o:(.bss+0x40): first defined here

pkgs/applications/window-managers/yeahwm/default.nix

@@ -24,6 +24,11 @@ stdenv.mkDerivation rec {
                       prefix="${placeholder "out"}" )
   '';
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: screen.o:(.bss+0x40): multiple definition of `fg'; client.o:(.bss+0x40): first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   postInstall = ''
     gzip -9 --stdout yeahwm.1 > yeahwm.1.gz
     install -m644 yeahwm.1.gz ${placeholder "out"}/share/man/man1/