[boost] [config] config.requires and simple_run

AMDG I've just been trying to get MinGW working without requiring the compiler's bin directory to be in the PATH. It mostly works now, but unfortunately I ran into problems with simple_run_action. The problem essentially is that this rule ignores RUN_PATH, which may be set by some toolsets (a quick grep shows borland, cw, gcc, and intel) to indicate additional paths that must be added to PATH when running the executable. In addition, I'm wondering how this is supposed to work with a cross-compiler. We really shouldn't be requiring cross-compilers to have access to the target system for ordinary builds. In Christ, Steven Watanabe