libredirect: Use extensions.sharedLibrary

This is to make sure we get the correct shared library suffix of the
target platform. While for example on Darwin it would even work with the
hardcoded .so prefix it's IMHO a bit nicer to have the actual native
extension.

Signed-off-by: aszlig <aszlig@nix.build>

pkgs/build-support/libredirect/default.nix

@@ -5,11 +5,16 @@ stdenv.mkDerivation {
 
   unpackPhase = "cp ${./libredirect.c} libredirect.c";
 
+  shlibext = stdenv.targetPlatform.extensions.sharedLibrary;
+
   buildPhase = ''
-    $CC -Wall -std=c99 -O3 -shared libredirect.c -o libredirect.so -fPIC -ldl
+    $CC -Wall -std=c99 -O3 -shared libredirect.c \
+      -o "libredirect$shlibext" -fPIC -ldl
   '';
 
-  installPhase = "mkdir -p $out/lib; cp libredirect.so $out/lib";
+  installPhase = ''
+    install -vD "libredirect$shlibext" "$out/lib/libredirect$shlibext"
+  '';
 
   meta = {
     platforms = stdenv.lib.platforms.unix;