libre: use cmake for configurePhase

Although libre is a cmake-based project, our expression for this
library does not currently use cmake for the configurePhase.  This
commit causes it to do so.

This is necessary to get downstream libre and baresip to cross-compile.

pkgs/development/libraries/libre/default.nix

@@ -1,4 +1,6 @@
-{ lib, stdenv, fetchFromGitHub, zlib, openssl }:
+{ lib, stdenv, fetchFromGitHub, zlib, openssl
+, cmake }:
+
 stdenv.mkDerivation rec {
   version = "2.9.0";
   pname = "libre";
@@ -9,6 +11,7 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-YNAfHmohMqGGF8N/VdndJJ32PF/GMBoNtjo/t2lt6HA=";
   };
   buildInputs = [ zlib openssl ];
+  nativeBuildInputs = [ cmake ];
   makeFlags = [ "USE_ZLIB=1" "USE_OPENSSL=1" "PREFIX=$(out)" ]
     ++ lib.optional (stdenv.cc.cc != null) "SYSROOT_ALT=${stdenv.cc.cc}"
     ++ lib.optional (stdenv.cc.libc != null) "SYSROOT=${lib.getDev stdenv.cc.libc}"