Allow indication of the Boost.Build location with an argument. This allows to override the built-in BB when testing.

[SVN r41260]

boost-build.jam

@@ -1,10 +1,11 @@
-#~ Copyright (C) 2002-2003, David Abrahams.
-#~ Copyright (C) 2002-2003, Vladimir Prus.
-#~ Copyright (C) 2003, Rene Rivera.
+#~ Copyright (C) 2002-2003 David Abrahams.
+#~ Copyright (C) 2002-2003 Vladimir Prus.
+#~ Copyright (C) 2003,2007 Rene Rivera.
 #~ Use, modification and distribution are subject to the
 #~ Boost Software License, Version 1.0. (See accompanying file
 #~ LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
 BOOST_ROOT = $(.boost-build-file:D) ;
-boost-build tools/build/v2 ;
-
+BOOST_BUILD = [ MATCH --boost-build=(.*) : $(ARGV) ] ;
+BOOST_BUILD ?= tools/build/v2 ;
+boost-build $(BOOST_BUILD) ;