dmd: set --sysroot=/ to avoid cc-wrapper value

After NixOS/nixpkgs#210004 `dmd` started failing
build as:

    ld: warning: libm.so.6, needed by ./generated/linux/release/64/lib.so, not found (try using -rpath or -rpath-link)
    ld: /build/druntime/generated/linux/release/64/libdruntime.so: undefined reference to `log10@GLIBC_2.2.5'

This happens because --sysroot=/nix/store/does/not/exist removes not
just include headers by prefixing wrong path, but also removes RUNPATH
dependencies of linked libraries. It's an unintended effect.

Restore the build by reversing the effect with --sysroot=/.

pkgs/dmd/generic.nix

@@ -151,6 +151,13 @@ stdenv.mkDerivation rec {
     git
   ];
 
+  # Workaround cc-wrapper's --sysroot= value for `staging-next`: it
+  # breaks library lookup via RUNPATH:
+  #   ld: warning: libm.so.6, needed by ./generated/linux/release/64/lib.so, not found (try using -rpath or -rpath-link)
+  #   ld: /build/druntime/generated/linux/release/64/libdruntime.so: undefined reference to `log10@GLIBC_2.2.5'
+  # TODO(trofi): remove the workaround once cc-wrapper is fixed.
+  NIX_CFLAGS_COMPILE = [ "--sysroot=/" ];
+
   buildInputs = [
     curl
     tzdata