gnum4: Fix impurity in shell handling

If gnum4 is built outside of a chroot, it will decide to use
/run/current-system/sw/bin/sh as the shell for "syscmd". (It gets this
path via "getconf PATH". Maybe our Glibc shouldn't return that path,
at least not during Nix builds...) If such a build of gnum4 is
subsequently used *inside* a chroot, it won't work because
/run/current-system doesn't exist. So specify an explicit path to the
shell.

pkgs/development/tools/misc/gnum4/default.nix

@@ -1,4 +1,4 @@
-{stdenv, fetchurl}:
+{ stdenv, fetchurl }:
 
 stdenv.mkDerivation rec {
   name = "gnum4-1.4.17";
@@ -12,6 +12,8 @@ stdenv.mkDerivation rec {
     && !stdenv.isCygwin                    # XXX: `test-dup2' fails on Cygwin
     && !stdenv.isSunOS;                    # XXX: `test-setlocale2.sh' fails
 
+  configureFlags = "--with-syscmd-shell=${stdenv.shell}";
+
   # Upstream is aware of it; it may be in the next release.
   patches = [ ./s_isdir.patch ];