pptp: Fix shebangs

- Remove `which` as build-time input
- Use substituteInPlace instead of sed (warns if it fails to replace)
- Use perl in buildInputs as it's a run-time dependency
- Patch shebangs manually, not sure why fixupPhase doesn't do it

pkgs/tools/networking/pptp/default.nix

@@ -1,26 +1,29 @@
-{ stdenv, fetchurl, perl, ppp, iproute, which }:
+{ stdenv, fetchurl, perl, ppp, iproute }:
 
 stdenv.mkDerivation rec {
-  name = "pptp-${version}";
+  pname = "pptp";
   version = "1.10.0";
 
   src = fetchurl {
-    url = "mirror://sourceforge/pptpclient/${name}.tar.gz";
+    url = "mirror://sourceforge/pptpclient/${pname}-${version}.tar.gz";
     sha256 = "1x2szfp96w7cag2rcvkdqbsl836ja5148zzfhaqp7kl7wjw2sjc2";
   };
 
-  patchPhase =
-    ''
-      sed -e 's/install -o root/install/' -i Makefile
-    '';
-  preConfigure =
-    ''
-      makeFlagsArray=( IP=${iproute}/bin/ip PPPD=${ppp}/sbin/pppd \
-                       BINDIR=$out/sbin MANDIR=$out/share/man/man8 \
-                       PPPDIR=$out/etc/ppp )
-    '';
+  prePatch = ''
+    substituteInPlace Makefile --replace 'install -o root' 'install'
+  '';
 
-  nativeBuildInputs = [ perl which ];
+  preConfigure = ''
+    makeFlagsArray=( IP=${iproute}/bin/ip PPPD=${ppp}/sbin/pppd \
+                     BINDIR=$out/sbin MANDIR=$out/share/man/man8 \
+                     PPPDIR=$out/etc/ppp )
+  '';
+
+  buildInputs = [ perl ];
+
+  postFixup = ''
+    patchShebangs $out
+  '';
 
   meta = with stdenv.lib; {
     description = "PPTP client for Linux";