NTLM: check for features with #ifdef instead of #if

Feature defines are normally checked with #ifdef instead of #if in the rest of
the codebase. Additionally, some compilers warn when a macro is implicitly
evaluated to 0 because it is not defined, which was the case here.

Ref: https://github.com/curl/curl/pull/1362#discussion_r108605101
Closes https://github.com/curl/curl/pull/1367