autogen.sh: fix configuration without autotools, and cleanup (bgo#706497)

Configuring without autotools was broken (because the "hack" in place to
force autotools if NOCONFIGURE is specified, didn't take in account the
dependencies for using autotools). Besides, the code to check this has
been slightly improved:
- Before, it was a bit confusing ("have_autotools" and "enable_autotools"
variables didn't express full meaning of how they were being used)
- It was barely readable (using $TRUE and $FALSE variables, instead of 0/1,
helps a bit).
- The dependencies of autotools now give suggestions of package names that
could contain them.
autogen.sh: Fix NOCONFIGURE with_autotools check

As the comment above the check mentions, check whether NOCONFIGURE is
set, not whether it was empty. Introduced by
ea9c867401e04f2537901e8ec09d7be26d2c6c6e.
autogen.sh: When forcing autotools, ignore dependencies

This should fix the gnome-continuous build.