Fixed bugs and tweaked some build scripts.

* jamroot: Added dependencies on the backend GCC for Clang.
* boost/jamfile: 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.
* 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.