build: Fixed no NTLM support for email when CURL_DISABLE_HTTP is defined

USE_NTLM would only be defined if: HTTP support was enabled, NTLM and
cryptography weren't disabled, and either a supporting cryptography
library or Windows SSPI was being compiled against.

This means it was not possible to build libcurl without HTTP support
and use NTLM for other protocols such as IMAP, POP3 and SMTP. Rather
than introduce a new SASL pre-processor definition, removed the HTTP
prerequisite just like USE_SPNEGO and USE_KRB5.

Note: Winbind support still needs to be dependent on CURL_DISABLE_HTTP
as it is only available to HTTP at present.

This bug dates back to August 2011 when I started to add support for
NTLM to SMTP.