ledger: enable python command

ledger in nixpkgs currently uses python (it’s in buildInputs), but the
ledger python command isn’t available until you turn it on via a cmake
flag.

pkgs/applications/office/ledger/default.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchFromGitHub, cmake, boost, gmp, mpfr, libedit, python
-, texinfo, gnused }:
+, texinfo, gnused, usePython ? true }:
 
 stdenv.mkDerivation rec {
   name = "ledger-${version}";
@@ -19,7 +19,7 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
 
-  cmakeFlags = [ "-DCMAKE_INSTALL_LIBDIR=lib" ];
+  cmakeFlags = [ "-DCMAKE_INSTALL_LIBDIR=lib" (stdenv.lib.optionalString usePython "-DUSE_PYTHON=true") ];
 
   # Skip byte-compiling of emacs-lisp files because this is currently
   # broken in ledger...