mwprocapture: fix build with kernels <4.13

also do some minor cleanup

pkgs/os-specific/linux/mwprocapture/default.nix

@@ -1,7 +1,9 @@
 { stdenv, fetchurl, kernel, alsaLib }:
 
+with stdenv.lib;
+
 # The Magewell Pro Capture drivers are not supported for kernels older than 3.2
-assert stdenv.lib.versionAtLeast kernel.version "3.2.0";
+assert versionAtLeast kernel.version "3.2.0";
 
 # this package currently only supports x86 and x86_64, as I have no ARM device to test on
 assert (stdenv.system == "x86_64-linux") || (stdenv.system == "i686-linux");
@@ -11,7 +13,7 @@ let
   if stdenv.is64bit then "64"
   else "32";
 
-  libpath = stdenv.lib.makeLibraryPath [ stdenv.cc.cc stdenv.glibc alsaLib ];
+  libpath = makeLibraryPath [ stdenv.cc.cc stdenv.glibc alsaLib ];
 
 in
 stdenv.mkDerivation rec {
@@ -23,7 +25,7 @@ stdenv.mkDerivation rec {
     sha256 = "1arwnwrq52rs8g9zfxw8saip40vc3201sf7qnbqd2p23h8vzwb8i";
   };
 
-  patches = [ ./linux_4_13_fix.patch ];
+  patches = [] ++ optional (versionAtLeast kernel.version "4.13") ./linux_4_13_fix.patch;
 
   preConfigure =
   ''
@@ -58,7 +60,7 @@ stdenv.mkDerivation rec {
       "$out"/bin/mwcap-info
   '';
 
-  meta = with stdenv.lib; {
+  meta = {
     homepage = http://www.magewell.com/;
     description = "Linux driver for the Magewell Pro Capture family";
     license = licenses.unfreeRedistributable;