Fixed #29.

* jamroot: Added support for `--with-gcc-check` and `--without-gcc-check`
  command line options. They are used to configure whether `make check` runs
  in GCC build.
* gcc-release-versions.sh: The script often fails. So, added `--retry`
  option for `curl` in GCC version extraction to make it more stable.
* gcc/jamfile: Added support for GCC build without `make check`.
* gcc/install.sh: Likewise.