Added multi-configuration support.

* .gitignore: Fixed `.gitignore` to ignore `objdir`.
* compilers.jam:
    * Removed `pushd` and `popd` because they are bash-specific builtins.
    * Fixed invocation of ICC `compilervars.sh`.
    * `Fixed bugs in calculation of triplets.
* jamroot:
    * Removed `unspecified` value from `<intro-address-model>` feature. The
      default value for the feature is the system default bits (e.g., `64`
      for `x86_64-*`, `32` for `i686-*`, and so on).
    * Added dependencies on the backend GCC for Clang.
    * Fixed typos.
* gcc/jamfile:
    * Removed `ln -s lib lib64` because the convention of library
      directories differs from distribution to distribution and `lib`
      directory cannot be assumed to be the directory for 64-bit libraries.
    * Added a new GCC mirror site in Japan.
* icc/jamfile: Added logfile output.
* template/x86_64-unknown-linux-gnu/gcc-wrapper:
    * Use `-###` (dry run) option to robustly detect whether the target is
      64-bit or 32-bit.
    * Added workaround for Debian multiarch. Because GCC without Debian
      patches cannot find by default libraries that reside in Debian
      multiarch directories, `-rpath-link` options to the directories are
      required for GCC (and an underlying linker) to correctly resolve
      indirect dependencies among shared libraries in link time.
* template/x86_64-unknown-linux-gnu/g++-wrapper:
    * Use `-###` (dry run) option to robustly detect whether the target is
      64-bit or 32-bit.
    * Added workaround for Debian multiarch. Because GCC without Debian
      patches cannot find by default libraries that reside in Debian
      multiarch directories, `-rpath-link` options to the directories are
      required for GCC (and an underlying linker) to correctly resolve
      indirect dependencies among shared libraries in link time.
* template/x86_64-unknown-linux-gnu/site-config.jam:
    * Added support for ICC.
    * Just tweaked quotations etc.
* template/x86_64-unknown-linux-gnu/user-config.jam: Added support for ICC.
* mpfr/jamfile: Added `--with-gmp` option.
* mpc/jamfile: Added `--with-mpfr` and `--with-gmp` options.
* isl/latest.sh: Modified to use the new repository of ISL.
* isl/jamfile:
    * Modified to use the new repository of ISL.
    * Added `--with-gmp-prefix` option.
* cloog/jamfile: Added `--with-isl-prefix` and `--with-gmp-prefix` options.
* icu4c/jamfile: Removed `--enable-threads` or `--disable-threads` from the
  command line options for ICU4C configure script because they are no longer
  valid in recent versions of ICU4C.
* openmpi/jamfile: Removed `<intro-address-model>unspecified` property.
* boost/jamfile:
    * Removed `<intro-address-model>unspecified` property.
    * Introduced dependencies on GMP and MPFR build processes because they
      may be used by Boost.Multiprecision as its backend.
    * Reduced build concurrency for the Boost.Thread test suite in order to
      allow more concurrency per a unit test.
    * Added support for ICC.
    * Added `ulimit` for the size of core files. Because many Boost test
      cases crash, core files should be prevented from occupying large
      portion of disk space.
    * Added `ulimit` for Boost test suite.
    * Added selections of suitable concurrency for Boost test suite.
    * Tweaked the limit on CPU time for Boost test suite, because the test
      suite for Boost.Algorithm and Boost.Thread takes more than 600
      seconds.
* boost/toolset.sh: Added support for ICC.
* valgrind/jamfile: Fixed a wrong value for `--with-mpicc` configure option.
* clang/jamfile:
    * Removed a workaround for a bug appeared in recent revisions.
    * Added support of extra clang tools repository.
* summary.sh:
    * Removed the column for `address-model=unspecified`.
    * Added rows for Boost.Atomic, Boost.Coroutine, Boost.Lockfree and
      Boost.Multiprecision.
* Fixed #8.
* Fixed #17.
* Fixed #24.