Alter wrap_gmp_sprintf and wrap_gmp_snprintf to take an extra arg (length of buffer to which we write).
Remove Rmpq_sprintf_ret and Rmpq_snprintf_ret. (These now seem pointless in view of the changes to wrap_gmp_sprintf and wrap_gmp_snprintf.)
Add Math::GMPq::__GNU_MP_RELEASE.
Alter V.xs to accommodate the possibility that the global constant gmp_version has not been set in gmp.h. I've assumed this can only happen if the gmp library version is earlier than 4.0.0. (The gmp ChangeLog indicates that this is so.)
No longer include inttypes.h on the basis of USE_LONG_DOUBLE being defined. (Was harmless ... but didn't make much sense.)
