p4est: fixed versioning (#180737)

pkgs/development/libraries/science/math/p4est/default.nix

@@ -1,5 +1,8 @@
-{ lib, stdenv, fetchFromGitHub
-, autoreconfHook, pkg-config
+{ lib
+, stdenv
+, fetchFromGitHub
+, autoreconfHook
+, pkg-config
 , p4est-withMetis ? true, metis
 , p4est-sc
 }:
@@ -31,9 +34,11 @@ stdenv.mkDerivation {
   postPatch = ''
     sed -i -e "s:\(^\s*ACLOCAL_AMFLAGS.*\)\s@P4EST_SC_AMFLAGS@\s*$:\1 -I ${p4est-sc}/share/aclocal:" Makefile.am
   '';
-  preConfigure = ''
+  preAutoreconf = ''
     echo "2.8.0" > .tarball-version
-    ${if mpiSupport then "unset CC" else ""}
+  '';
+  preConfigure = lib.optionalString mpiSupport ''
+    unset CC
   '';
 
   configureFlags = p4est-sc.configureFlags