solc: remove Z3 version hardcoding

solc compiles with the latest Z3.

pkgs/by-name/so/solc/package.nix

@@ -10,7 +10,7 @@
   python3,
   versionCheckHook,
   z3Support ? true,
-  z3_4_11 ? null,
+  z3 ? null,
   cvc4Support ? gccStdenv.hostPlatform.isLinux,
   cvc4 ? null,
   cln ? null,
@@ -19,10 +19,6 @@
 
 # compiling source/libsmtutil/CVC4Interface.cpp breaks on clang on Darwin,
 # general commandline tests fail at abiencoderv2_no_warning/ on clang on NixOS
-let
-  z3 = z3_4_11;
-in
-
 assert z3Support -> z3 != null && lib.versionAtLeast z3.version "4.11.0";
 assert cvc4Support -> cvc4 != null && cln != null && gmp != null;
 
@@ -125,6 +121,8 @@ let
           ))
         ]; # contextlib2 glob2 textwrap3 traceback2 urllib3
 
+        enableParallelBuilding = true;
+
         # tests take 60+ minutes to complete, only run as part of passthru tests
         doCheck = false;