serf: support cross compilation

serf consumes apr-util's apu-1-config file to obtain linker flags, so we extend the existing Makefile patch there to also apply to apu-1-config

pkgs/development/libraries/apr-util/default.nix

@@ -48,6 +48,8 @@ stdenv.mkDerivation rec {
     lib.optionalString (stdenv.buildPlatform != stdenv.hostPlatform) ''
       substituteInPlace Makefile \
         --replace "-ldb-6.9" "-ldb"
+      substituteInPlace apu-1-config \
+        --replace "-ldb-6.9" "-ldb"
   '';
 
   propagatedBuildInputs = [ apr expat libiconv libxcrypt ]

pkgs/development/libraries/serf/scons.patch

@@ -11,10 +11,11 @@ index 4358a23..6ce7776 100644
      if target in build_targets:
        return PathVariable.PathIsDirCreate(key, val, env)
      else:
-@@ -155,6 +155,7 @@ if sys.platform == 'win32':
+@@ -155,6 +155,8 @@ if sys.platform == 'win32':
  env = Environment(variables=opts,
                    tools=('default', 'textfile',),
                    CPPPATH=['.', ],
++                  AR=os.environ['AR'],
 +                  ENV=os.environ,
                    )