ARGV rule

[david_abrahams]

The ARGV builtin rule, which returns the arguments 
with which Jam was invoked. $(ARGV[1]) can be used to 
find the name that was used to invoke Jam, which can 
be used as a key to whether to implement stock 
Perforce Jam behavior or Boost.Build behavior. 

[René Rivera]

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user_id=33595

This is in, and work in Linux/gcc. Should work in anything
else, but testing would be nice.