pcsclite: Introduce PCSCLITE_HP_DROPDIR env var

This allows to override the directory where PCSC-Lite searches for USB
drivers at runtime and should make our NixOS module much more clean so
that we don't need to imperatively stitch together plugin directories
anymore.

I'm using a GNU extension for the ternary operator to avoid computing
getenv("PCSCLITE_HP_DROPDIR") twice:

https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html#Conditionals

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @viric, @wkennington

pkgs/tools/security/pcsclite/default.nix

@@ -8,6 +8,8 @@ stdenv.mkDerivation rec {
     sha256 = "12k8q0ckyy1fqcfh7x0b7kfrlfiscrqaqmidcggnzs4pi2iqml77";
   };
 
+  patches = [ ./no-dropdir-literals.patch ];
+
   configureFlags = [
     # The OS should care on preparing the drivers into this location
     "--enable-usbdropdir=/var/lib/pcsc/drivers"
@@ -15,6 +17,12 @@ stdenv.mkDerivation rec {
   ] ++ stdenv.lib.optional stdenv.isLinux
          "--with-systemdsystemunitdir=\${out}/etc/systemd/system";
 
+  postConfigure = ''
+    sed -i -re '/^#define *PCSCLITE_HP_DROPDIR */ {
+      s/(DROPDIR *)(.*)/\1(getenv("PCSCLITE_HP_DROPDIR") ? : \2)/
+    }' config.h
+  '';
+
   nativeBuildInputs = [ pkgconfig perl python2 ];
   buildInputs = stdenv.lib.optionals stdenv.isLinux [ udev dbus_libs ];