sngrep: fix error if gnutls and openssl are both enabled

sngrep: fix error if gnutls and openssl are both enabled

gnutls and openssl support can't be enabled at the same time in
sngrep. Prefer openssl if available, otherwise use gnutls.

Fixes:

  http://autobuild.buildroot.net/results/7c775b1faf7be90fb4acd3653600f7b6b48aa86f/

Signed-off-by: Adam Duskett <aduskett@codeblue.com>
[Thomas:
 - regroup CONF_OPTS lines
 - add an 'else' clause to explicitly disable gnutls and openssl when
   none are available
 - add a reference to the autobuilder failure]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>