eukleides: add -fcommon workaround

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

    ld: eukleides_build/triangle.o:(.bss+0x28): multiple definition of `A';
        eukleides_build/quadrilateral.o:(.bss+0x18): first defined here

pkgs/applications/science/math/eukleides/default.nix

@@ -32,6 +32,12 @@ lib.fix (eukleides: stdenv.mkDerivation rec {
       --replace '$(SHARE_DIR)/texmf' "$tex"
   '';
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: eukleides_build/triangle.o:(.bss+0x28): multiple definition of `A';
+  #     eukleides_build/quadrilateral.o:(.bss+0x18): first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   preInstall = ''
     mkdir -p $out/bin
   '';