Added support for ICC.

* compilers.jam: Fixed invocation of ICC `compilervars.sh`.
* icc/jamfile: Added logfile output.
* template/x86_64-unknown-linux-gnu/site-config.jam: Added support for ICC.
* template/x86_64-unknown-linux-gnu/user-config.jam: Added support for ICC.
* boost/jamfile: Added support for ICC.
* boost/toolset.sh: Added support for ICC.