libssl compatibility issue

[muesli]

Trying to compile frivpn on Debian stretch, I get this error:

gcc -Wall -g -fPIE -D_FORTIFY_SOURCE=2 -fstack-protector-strong -fwrapv --param ssp-buffer-size=4 -shared -fPIC -c -o ovpn.o ovpn.c -I/usr/include/lua5.2
In file included from ovpn.c:26:0:
ovpn.h:42:17: error: field ‘evp_dec’ has incomplete type
  EVP_CIPHER_CTX evp_dec;
                 ^~~~~~~
ovpn.h:43:17: error: field ‘evp_enc’ has incomplete type
  EVP_CIPHER_CTX evp_enc;
                 ^~~~~~~
ovpn.h:44:11: error: field ‘hmac_tx’ has incomplete type
  HMAC_CTX hmac_tx;
           ^~~~~~~
ovpn.h:45:11: error: field ‘hmac_rx’ has incomplete type
  HMAC_CTX hmac_rx;
           ^~~~~~~
ovpn.h:84:11: error: field ‘hmac_tx’ has incomplete type
  HMAC_CTX hmac_tx;
           ^~~~~~~
ovpn.h:85:11: error: field ‘hmac_rx’ has incomplete type
  HMAC_CTX hmac_rx;
           ^~~~~~~
ovpn.c: In function ‘ctl_init’:
ovpn.c:104:2: warning: implicit declaration of function ‘HMAC_CTX_init’ [-Wimplicit-function-declaration]
  HMAC_CTX_init(&ctl->hmac_tx);
  ^~~~~~~~~~~~~
Makefile:18: recipe for target 'ovpn.o' failed
make: *** [ovpn.o] Error 1

Digging a bit deeper, this seems to be an issue with newer libssl versions: openssl/openssl#962