libiconv: Make setup hook work for cross

pkgs/development/libraries/libiconv/setup-hook.sh

@@ -1,7 +1,18 @@
 # libiconv must be listed in load flags on non-Glibc
 # it doesn't hurt to have it in Glibc either though
 iconvLdflags() {
-    export NIX_LDFLAGS="$NIX_LDFLAGS -liconv"
+    # The `depHostOffset` describes how the host platform of the dependencies
+    # are slid relative to the depending package. It is brought into scope of
+    # the environment hook defined as the role of the dependency being applied.
+    case $depHostOffset in
+        -1) local role='BUILD_' ;;
+        0)  local role='' ;;
+        1)  local role='TARGET_' ;;
+        *)  echo "cc-wrapper: Error: Cannot be used with $depHostOffset-offset deps" >2;
+            return 1 ;;
+    esac
+
+    export NIX_${role}LDFLAGS+=" -liconv"
 }
 
 addEnvHooks "$hostOffset" iconvLdflags