Add configure option --disable-doc

* Makefile.am (doc) [!BUILD_DOC]: Do not recurse into doc/.
* configure.ac (BUILD_DOC): New am_conditional and new option.

We have this option already for GnuPG and should use it for all
projects. Embedded platforms usually do not require documentation and
being able to disable the building avoids a lot of build dependencies.

• Suggested-by: Hans-Christoph Steiner <hans@guardianproject.info>