Simplify BOOST_ROOT setting.

[SVN r19981]

Jamrules

@@ -6,9 +6,8 @@
 # user on the command-line or in the environment
 
 # Establish this as the root of the boost installation. Most targets will want
-# $(BOOST_ROOT) in their #include path.  $(gTOP) is the name of the variable
-# containing the path from the invocation directory to the project root.
-path-global BOOST_ROOT : $($(gTOP)) ;
+# $(BOOST_ROOT) in their #include path.
+path-global BOOST_ROOT : . ;
 
 # The current version of Boost.
 BOOST_VERSION ?= 1.31.0 ;