Fixed bugs found in Ubuntu 10.04.

* `compilers.jam`: Fixed bugs in calculation of triplets.
* `template/x86_64-unknown-linux-gnu/gcc-wrapper`: Use `-###` (dry run)
  option to robustly detect whether the target is 64-bit or 32-bit.
* `template/x86_64-unknown-linux-gnu/g++-wrapper`: Likewise.
* `mpfr/jamfile`: Added `--with-gmp` option.
* `mpc/jamfile`: Added `--with-mpfr` and `--with-gmp` options.
* `isl/jamfile`: Added `--with-gmp-prefix` option.
* `cloog/jamfile`: Added `--with-isl-prefix` and `--with-gmp-prefix` options.
* `boost/jamfile`: Added `ulimit` for Boost test suite.