Implement variant declaration.

    * builtin.jam (variant): New rule.

    * build-system.jam: Unconditionally add <toolset>gcc to build
        request. Will be fixed when toolset support is ready.

    * feature.jam (extend-feature): Bugfix.


[SVN r15678]

tools/build

@@ -1 +1 @@
-Subproject commit 18991908fc12a7e2b07026cb21dd8c1f71c5c68e
+Subproject commit 2d011cda1741d839d5533052ad9604d23f6c24d6