Add provisions to build with Libgcrypt 1.6.

Replace gcry_md_start_debug by gcry_md_debug in all files.

• agent/gpg-agent.c (fixed_gcry_pth_init): Use only if

GCRY_THREAD_OPTION_VERSION is 0

• scd/scdaemon.c (fixed_gcry_pth_init): Ditto. --

Libgcrypt 1.6 will have some minor API changes. In particular some
deprecated macros and functions will be removed. PTH will also be
dropped in favor of a thread model neutral locking method.