https://bugs.webkit.org/show_bug.cgi?id=120424

Reviewed by Martin Robinson.

The maintainer mode feature is used by ostree and other automated builders to ensure no autotools
regeneration will happen for a regular tarball build; ostree builders, for instance, are very
conservative with toolchain upgrades, and are still using aclocal 1.12. WebKit's latest tarball
(2.1.90) for some reason tries to regenerate build files, and the build fails because it can't find
the version of aclocal that was used for generating the tarball (1.13).

* configure.ac: enable maintainer mode feature.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154787 268f45cc-cd09-0410-ab3c-d52691b4dbfc