libcap: Fix cross-compilation

pkgs/os-specific/linux/libcap/default.nix

···
-{ stdenv, fetchurl, attr, perl, pam ? null }:
+{ stdenv, buildPackages, fetchurl, attr, perl, pam ? null }:
 assert pam != null -> stdenv.isLinux;
 
 stdenv.mkDerivation rec {
···
   outputs = [ "out" "dev" "lib" "man" "doc" ]
     ++ stdenv.lib.optional (pam != null) "pam";
 
-  nativeBuildInputs = [ perl ];
+  nativeBuildInputs = [ perl buildPackages.stdenv.cc ];
 
   buildInputs = [ pam ];
 
···
   makeFlags = [
     "lib=lib"
     (stdenv.lib.optional (pam != null) "PAM_CAP=yes")
+    "BUILD_CC=${buildPackages.stdenv.cc.targetPrefix}gcc"
+    "CC=${stdenv.cc.targetPrefix}gcc"
   ];
 
   prePatch = ''