mbedtls: switch off NTLM in build if md4 isn't available

NTLM support with mbedTLS was added in 497e7c9d but requires that mbedTLS
is built with the MD4 functions available, which it isn't in default
builds. This now adapts if the funtion isn't there and builds libcurl
without NTLM support if so.

Fixes #1004