Disabled C++ support for GMP.

- gmp/jamfile: C++ support for GMP is disabled because it
  - results in build errors with GCC 4.9,
  - results in an error during `make check` with GCC and LTO, and
  - can be replaced by `Boost.Multiprecision`, which adopts more modern
    C++ programming styles.
Merge branch 'master' into fix/gmp
Merge pull request #57 from Cryolite/fix/gmp

Disabled C++ support for GMP.