Fix setting of BOOST_BUILD_PATH.

[SVN r22634]

tools/regression/run_tests.sh

@@ -85,7 +85,11 @@ boost_build_path="$HOME/.boost-build"
 # Some setup.
 #
 boost_dir=`basename "$boost_root"`
-export BOOST_BUILD_PATH="$boost_build_path" "$BOOST_BUILD_PATH"
+if test -n "${BOOST_BUILD_PATH}" ; then
+    export BOOST_BUILD_PATH="$boost_build_path:$BOOST_BUILD_PATH"
+else
+    export BOOST_BUILD_PATH="$boost_build_path"
+fi
 
 #
 # STEP 0: