gtls: Fixed compilation when GnuTLS < 3.5.0

[captain-caveman2k]

Commit 41fcb4f from #4958 breaks compiling with GnuTLS older than 3.5.0 (Specifically 3.4.10) with the following error:

/curl/lib/vtls/gtls.c:667:32: error: 'GNUTLS_FORCE_CLIENT_CERT' undeclared (first use in this function)
init_flags = GNUTLS_CLIENT | GNUTLS_FORCE_CLIENT_CERT;

See this autobuild for more details.

With this fix, curl builds successfully and executes runtest.pl with the following summary:

Warning: dict server unexpectedly alive
Warning: smb server unexpectedly alive
TESTDONE: 1247 tests out of 1247 reported OK: 100%
TESTDONE: 1319 tests were considered during 462 seconds.
make[1]: Leaving directory '/home/steve/Development/cc2k/tests'

[jay]

LGTM except the commit subject should be in the imperative. travis isn't showing in the CI not sure what that's about.

[bagder]

Maybe just do #ifdef as that's how we normally check for the presence of a single define.

[captain-caveman2k]

Whilst I'm the first to admit that I'm a fully fledged member of the #if everywhere club, I do "try" and use the style of the code that is around me.

As such I noted the following on line 673 and kept the styling the same:

#if defined(GNUTLS_NO_TICKETS)

[bagder]

I think "fixing the code while changing it" is better than sticking to the "wrong" style...