* fixing some comments
* fixed regex and condition in resolve-reference
* removed common-properties2 rule
* fixed regex in basic-target.compute-usage-requirements
* fixed xusage-requirements in basic-target.compute-usage-requirements
 now is a local variable instead of a class attribute
* removed all checking for '@error', not need anymore since
 0419f26 (2003年09月26日), this
 is also done in build/project.jam and build/property-set.jam
* emit error using errors module in typed-target.construct
* removed unused imports