libftdi: enable async mode

Apparently, async mode for libftdi 0.20 is allegedly disabled when using
libusb-compat wrappers, as libftdi does not really support libftdi 1.x. Because
we only ship libusb-compat, this would normally make async mode completely
unavailable.

Except distributions like Ubuntu just disable this check completely! See this
patch from Launchpad:

https://bazaar.launchpad.net/~ubuntu-branches/ubuntu/trusty/libftdi/trusty/view/head:/debian/patches/04_async_mode.diff

In the spirit of good competition (and feature parity for software that
*doesn't* support the synchronous mode, thanks to Ubuntu silently ensuring
their async-only paths work) we enable this just the same.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

pkgs/development/libraries/libftdi/default.nix

@@ -14,6 +14,14 @@ stdenv.mkDerivation rec {
 
   # Hack to avoid TMPDIR in RPATHs.
   preFixup = ''rm -rf "$(pwd)" '';
+  configureFlags = [ "--with-async-mode" ];
+
+  # allow async mode. from ubuntu. see:
+  #   https://bazaar.launchpad.net/~ubuntu-branches/ubuntu/trusty/libftdi/trusty/view/head:/debian/patches/04_async_mode.diff
+  patchPhase = ''
+    substituteInPlace ./src/ftdi.c \
+      --replace "ifdef USB_CLASS_PTP" "if 0"
+  '';
 
   meta = {
     description = "A library to talk to FTDI chips using libusb";