firefoxPackages: support building with firefox 65

Firefox >=65 will depend on icu >=63. All the older firefox versions
(and derived packages) seem to work fine with this change.

Also the system path environment patch will fail to apply since there
was a trivial whitespace change in the source file. By adding `-l` to
patch we can avoid having to track two patches that do basically the
same. Having patchFlags per file without resorting to pre-/postPatch
would be nicer but there doesn't seem to be a facility for that right
now.

pkgs/applications/networking/browsers/firefox/common.nix

@@ -87,6 +87,11 @@ stdenv.mkDerivation rec {
 
   inherit src unpackPhase patches meta;
 
+  # Ignore trivial whitespace changes in patches, this fixes compatibility of
+  # ./env_var_for_system_dir.patch with Firefox >=65 without having to track
+  # two patches.
+  patchFlags = [ "-p1" "-l" ];
+
   buildInputs = [
     gtk2 perl zip libIDL libjpeg zlib bzip2
     dbus dbus-glib pango freetype fontconfig xorg.libXi xorg.libXcursor