gnu-apl: fix clang build

/cc ZHF #36454

pkgs/development/interpreters/gnu-apl/default.nix

@@ -1,6 +1,5 @@
 { stdenv, fetchurl, readline, gettext, ncurses }:
 
-with stdenv.lib;
 stdenv.mkDerivation rec {
   name = "gnu-apl-${version}";
   version = "1.7";
@@ -13,9 +12,10 @@ stdenv.mkDerivation rec {
   buildInputs = [ readline gettext ncurses ];
 
   # Needed with GCC 7
-  NIX_CFLAGS_COMPILE = "-Wno-error=int-in-bool-context";
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isGNU "-Wno-error=int-in-bool-context"
+    + stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=null-dereference";
 
-  patchPhase = optionalString stdenv.isDarwin ''
+  patchPhase = stdenv.lib.optionalString stdenv.isDarwin ''
     substituteInPlace src/LApack.cc --replace "malloc.h" "malloc/malloc.h"
   '';
 
@@ -24,7 +24,7 @@ stdenv.mkDerivation rec {
     find $out/share/doc/support-files -name 'Makefile*' -delete
   '';
 
-  meta = {
+  meta = with stdenv.lib; {
     description = "Free interpreter for the APL programming language";
     homepage    = http://www.gnu.org/software/apl/;
     license     = licenses.gpl3Plus;