Fixed #32.

* compilers.jam: Changed `-std=c++0x` option for GCC to `-std=c++11`. It
  implies that support for GCC 4.6.x has been removed.
* template/x86_64-unknown-linux-gnu/site-config.jam: Likewise.
* boost/jamfile: Likewise.
* awacs/clang-trunk/jamroot: Likewise.
* awacs/gcc-snapshot/jamroot: Likewise.
* ppl/jamfile: Changed `-std=gnu++0x` option for GCC to `-std=gnu++11`. It
  implies that support for GCC 4.6.x has been removed.