Debian: added configurable VAGRANTBOX env variable.

This should make it easy to build in different virtual
machines, e.g. 32-bit.

deb/Makefile

@@ -1,9 +1,10 @@
 COMMIT?=HEAD
+VAGRANTBOX?=ubuntu/trusty64
 
 .PHONY: package clean
 
 package:
-	vagrant up
+	VAGRANTBOX=$(VAGRANTBOX) vagrant up
 	vagrant ssh -c 'rm -rf pandoc && git clone https://github.com/jgm/pandoc && cd pandoc && git checkout -b work $(COMMIT) && git submodule update --init && ./make_deb.sh && cp *.deb /vagrant_data/'
 	vagrant halt
 

deb/Vagrantfile

@@ -12,7 +12,7 @@ Vagrant.configure(2) do |config|
 
   # Every Vagrant development environment requires a box. You can search for
   # boxes at https://atlas.hashicorp.com/search.
-  config.vm.box = "ubuntu/trusty64"
+  config.vm.box = ENV['VAGRANTBOX'] || "ubuntu/trusty64"
 
   # Disable automatic box update checking. If you disable this, then
   # boxes will only be checked for updates when the user runs