mbedtls: do not define mbedtls_ctr_drbg_update_ret when not needed

mbedtls: do not define mbedtls_ctr_drbg_update_ret when not needed

The mbedtls_ctr_drbg_update_ret() function was backported to various
older branches, including 2.14 and 2.7.
To avoid making the #if guard too complex, let's detect if this function
exist at configure time.
All versions not having this function, will use our compat code.

Cc: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Message-Id: <20210812085300.4738-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22734.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>