Optimization: speed up generator selection

From: Vladimir Prus (ghost_at_[hidden])
Date: 2004-12-17 10:12:39

Some caching again.

Details: generators.find-viable-generators is called many times to find 
suitable generator for a given target type and property-set. That information 
is very easy to cache.

Performance:

Femtown (reference Boost.Build)
Real: min 38:770, max 42:770, avg 40:660
User: min 34:40, max 35:110, avg 34:500

Femtown (CVS Boost.Build)
Real: min 40:70, max 46:490, avg 43:166
User: min 30:190, max 31:420, avg 30:820

TBLib (reference Boost.Build)
Real: min 17:780, max 18:400, avg 18:73
User: min 15:630, max 15:870, avg 15:746
TBLib (CVS Boost.Build)
Command is '/home/ghost/Work/boost/tools/build/jam_src/bin.linuxx86/bjam -n 
-d0'
Real: min 14:700, max 15:240, avg 14:923
User: min 13:200, max 13:300, avg 13:246

- Volodya