miranda: fix build with clang

pkgs/development/compilers/miranda/default.nix

···
   # Workaround build failure on -fno-common toolchains like upstream
   # gcc-10. Otherwise build fails as:
   #   ld: types.o:(.bss+0x11b0): multiple definition of `current_file'; y.tab.o:(.bss+0x70): first defined here
-  env.NIX_CFLAGS_COMPILE = "-fcommon";
+  env.NIX_CFLAGS_COMPILE = toString ([
+    "-fcommon"
+  ] ++ lib.optionals stdenv.cc.isClang [
+    "-Wno-error=int-conversion"
+  ]);
 
   makeFlags = [
     "CC=${stdenv.cc.targetPrefix}cc"