"./configure --help" displays "--disable-dauth disable HTTP basic and digest Auth support". However:
$ CFLAGS=-O3 ./configure --disable-postprocessor --disable-dauth --enable-https=no
(...)
configure: Configuration Summary:
Operating System: linux-gnu
Threading lib: posix
libcurl (testing): yes
Target directory: /usr/local
Messages: yes
Basic auth.: yes
Digest auth.: yes
Postproc: no
HTTPS support: no (disabled)
epoll support: yes
build docs: yes
build examples: yes
libmicrospdy: yes
spdylay (testing): no
I noticed that "--disable-bauth" works, but digest auth remains enabled.