Added --use-system-layout option to boost formula

I've added the '--use-system-layout' option to boost formula. It passes the '--layout=system' option instead of regular '--layout=tagged' to bjam and thus removes the '-mt' suffix from resulting binaries.

The case is, the default value of this option on unix-like systems (including os x) is 'system' as it is stated in bjam man page:
> The default value is 'versioned' on Windows, and
> 'system' on Unix.
It produces binaries without any suffixes (i. e. 'libboost_chrono.a' instead of 'libboost_chrono-mt.a') which I find very handy, because a lot of software that links with boost searches for libraries without any suffixes in their names.

The 'tagged' layout (which boost formula is using at the moment) is dedicated to be used in case of several variants of Boost mostly:
> tagged -- Names of boost binaries include the
> encoded build properties such as variant and
> threading, but do not including compiler name
> and version, or Boost version. This option is
> useful if you build several variants of Boost,
> using the same compiler.

I see no reason in limiting the layout to 'tagged' only.