Support LINK_ONLY in order to parallel the Boost Build link/link-fail rules

[eldiener]

Since you have support in for COMPILE_ONLY in the bcm_test for paralleling the compile/compile-fail rules of Boost Build you can also add support for Boost Build link/link-fail rules with a LINK_ONLY option. You would implement this like the COMPILE_ONLY but in this case your target would be an executable with an EXCLUDE_FROM_ALL and your test would simply be an attempt to build the executable. If you like you could call this option BUILD_ONLY instead of LINK_ONLY, although the LINK_ONLY better mirrors the Boost Build rule name. Once this is done you would have covered the specializing testing rules of Boost Build explained at http://www.boost.org/build/doc/html/bbv2/builtins/testing.html.