libgcrypt: Update to 1.6.4

libgcrypt: Update to 1.6.4

* configure.ac: Change LT version to C20/A0/R4.

w32: Avoid a few compiler warnings.
* cipher/cipher-selftest.c (_gcry_selftest_helper_cbc)
(_gcry_selftest_helper_cfb, _gcry_selftest_helper_ctr): Mark variable
as unused.
* random/rndw32.c (slow_gatherer): Avoid signed pointer mismatch
warning.
* src/secmem.c (init_pool): Avoid unused variable warning.
* tests/random.c (writen, readn): Include on if needed.

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>