Optimization: avoid virtual-target.str calls

From: Vladimir Prus (ghost_at_[hidden])
Date: 2004-12-22 03:42:45

The relatively slow virtual-target.str method was invoked many times, even 
though it's needed only for --debug-generators.

Performance: -0.5 secs. Not bad for a one-line patch.

Femtown (reference Boost.Build)
Real: min 18:350, max 21:240, avg 19:516
User: min 17:730, max 18:180, avg 17:906

Femtown (CVS Boost.Build)
Real: min 18:280, max 19:30, avg 18:640
User: min 17:210, max 17:480, avg 17:326

TBLib (reference Boost.Build)
Real: min 10:10, max 10:520, avg 10:280
User: min 9:750, max 9:990, avg 9:863

TBLib (CVS Boost.Build)
Real: min 9:810, max 10:780, avg 10:160
User: min 9:560, max 10:120, avg 9:786