Ever since bug 969164, the js build system, when building gecko (not when
building standalone) uses a autoconf-js.mk file for its config.

One of the suboptimal ways we have to retrigger builds when the build
configuration changes (changes to e.g. configure.in can do that) is to
make most things depend on autoconf.mk. Which unfortunately doesn't
account for the fact the js/src subdirectory uses a different file.

In practice, this means that some classes of changes to the js build
system, not accompanied with toplevel build system changes may no
trigger the corresponding rebuilds in the js subtree on incremental
builds.