borgbackup: unbundle xxhash and use pkgconfig

Borg can use the python pkgconfig package to discover library paths, we
don't need to pass them explicitly.

Removes libb2, because borg since version 1.2.0 uses hashlib.blake2b
instead.

pkgs/tools/backup/borgbackup/default.nix

@@ -7,6 +7,7 @@
 , openssh
 , openssl
 , python3
+, xxHash
 , zstd
 , installShellFiles
 , nixosTests
@@ -41,6 +42,7 @@ python3.pkgs.buildPythonApplication rec {
   nativeBuildInputs = with python3.pkgs; [
     cython
     setuptools-scm
+    pkgconfig
 
     # docs
     sphinxHook
@@ -55,6 +57,7 @@ python3.pkgs.buildPythonApplication rec {
   buildInputs = [
     libb2
     lz4
+    xxHash
     zstd
     openssl
   ] ++ lib.optionals stdenv.isLinux [
@@ -67,13 +70,6 @@ python3.pkgs.buildPythonApplication rec {
     (if stdenv.isLinux then pyfuse3 else llfuse)
   ];
 
-  preConfigure = ''
-    export BORG_OPENSSL_PREFIX="${openssl.dev}"
-    export BORG_LZ4_PREFIX="${lz4.dev}"
-    export BORG_LIBB2_PREFIX="${libb2}"
-    export BORG_LIBZSTD_PREFIX="${zstd.dev}"
-  '';
-
   makeWrapperArgs = [
     ''--prefix PATH ':' "${openssh}/bin"''
   ];