build: Unset 'NIX_BUILD_HOOK' when offloading support is missing.

* build-aux/pre-inst-env.in: Export 'NIX_BUILD_HOOK' only when
nix/scripts/offload is executable; unset it otherwise.

diff --git a/build-aux/pre-inst-env.in b/build-aux/pre-inst-env.in
index fe56da6..dca2699 100644 (file)
--- a/build-aux/pre-inst-env.in
+++ b/build-aux/pre-inst-env.in
@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2012, 2013, 2014, 2015, 2017 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of GNU Guix.
 #
@@ -45,11 +45,18 @@ export PATH
 
 NIX_ROOT_FINDER="$abs_top_builddir/nix/scripts/list-runtime-roots"
 NIX_SUBSTITUTERS="$abs_top_builddir/nix/scripts/substitute"
-NIX_BUILD_HOOK="$abs_top_builddir/nix/scripts/offload"
 NIX_LIBEXEC_DIR="@abs_top_builddir@/nix/scripts" # for 'guix-authenticate'
 
-export NIX_ROOT_FINDER NIX_SUBSTITUTERS        \
-    NIX_BUILD_HOOK NIX_LIBEXEC_DIR
+export NIX_ROOT_FINDER NIX_SUBSTITUTERS NIX_LIBEXEC_DIR
+
+NIX_BUILD_HOOK="$abs_top_builddir/nix/scripts/offload"
+if [ -x "$NIX_BUILD_HOOK" ]
+then
+    export NIX_BUILD_HOOK
+else
+    # No offloading support.
+    unset NIX_BUILD_HOOK
+fi
 
 # The 'guix-register' program.
 GUIX_REGISTER="$abs_top_builddir/guix-register"