libredirect: introduce optional setup-hook

This allows to simplify the usage of libredirect inside of nix build
sandboxes. Add "libredirect.hook" to the build inputs to get everything
linked in automaticall. All that's left is to set NIX_REDIRECTS and call
the target program.

pkgs/build-support/libredirect/default.nix

@@ -10,6 +10,8 @@ stdenv.mkDerivation {
 
   libName = "libredirect" + stdenv.targetPlatform.extensions.sharedLibrary;
 
+  outputs = ["out" "hook"];
+
   buildPhase = ''
     $CC -Wall -std=c99 -O3 -shared libredirect.c \
       -o "$libName" -fPIC -ldl
@@ -21,17 +23,25 @@ stdenv.mkDerivation {
 
   installPhase = ''
     install -vD "$libName" "$out/lib/$libName"
+
+    mkdir -p "$hook/nix-support"
+    cat <<SETUP_HOOK > "$hook/nix-support/setup-hook"
+    ${if stdenv.isDarwin then ''
+    export DYLD_INSERT_LIBRARIES="$out/lib/$libName"
+    export DYLD_FORCE_FLAT_NAMESPACE=1
+    '' else ''
+    export LD_PRELOAD="$out/lib/$libName"
+    ''}
+    SETUP_HOOK
   '';
 
   doInstallCheck = true;
 
-  installCheckPhase = if stdenv.isDarwin then ''
-    NIX_REDIRECTS="/foo/bar/test=${coreutils}/bin/true" \
-    DYLD_INSERT_LIBRARIES="$out/lib/$libName" \
-    DYLD_FORCE_FLAT_NAMESPACE=1 ./test
-  '' else ''
-    NIX_REDIRECTS="/foo/bar/test=${coreutils}/bin/true" \
-    LD_PRELOAD="$out/lib/$libName" ./test
+  installCheckPhase = ''
+    (
+      source "$hook/nix-support/setup-hook"
+      NIX_REDIRECTS="/foo/bar/test=${coreutils}/bin/true" ./test
+    )
   '';
 
   meta = {