libsolv: Build without rpm dependency on darwin

RPM is no longer available for
Darwin ( NixOS#196350 ). Libsolv is used by downstream
packages such as micromamba which have no requirement on RPM.

pkgs/development/libraries/libsolv/default.nix

@@ -1,6 +1,9 @@
 { lib, stdenv, fetchFromGitHub, cmake, ninja, pkg-config
 , zlib, xz, bzip2, zchunk, zstd
-, expat, rpm, db }:
+, expat
+, withRpm ? !stdenv.isDarwin, rpm
+, db
+}:
 
 stdenv.mkDerivation rec {
   version  = "0.7.22";
@@ -15,19 +18,21 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = [
     "-DENABLE_COMPLEX_DEPS=true"
-    "-DENABLE_RPMMD=true"
-    "-DENABLE_RPMDB=true"
-    "-DENABLE_PUBKEY=true"
-    "-DENABLE_RPMDB_BYRPMHEADER=true"
     "-DENABLE_LZMA_COMPRESSION=true"
     "-DENABLE_BZIP2_COMPRESSION=true"
     "-DENABLE_ZSTD_COMPRESSION=true"
     "-DENABLE_ZCHUNK_COMPRESSION=true"
     "-DWITH_SYSTEM_ZCHUNK=true"
-  ];
+  ] ++ lib.optionals ( withRpm ) [
+    "-DENABLE_PUBKEY=true"
+    "-DENABLE_RPMDB=true"
+    "-DENABLE_RPMDB_BYRPMHEADER=true"
+    "-DENABLE_RPMMD=true"
+  ] ;
 
   nativeBuildInputs = [ cmake ninja pkg-config ];
-  buildInputs = [ zlib xz bzip2 zchunk zstd expat rpm db ];
+  buildInputs = [ zlib xz bzip2 zchunk zstd expat db ]
+                ++ lib.optionals ( withRpm ) [ rpm ];
 
   meta = with lib; {
     description = "A free package dependency solver";