build: Add --disable-doc option.

Authored by gouttegd on Nov 20 2018, 1:48 PM.

Description

build: Add --disable-doc option.

* configure.ac (BUILD_DOC): Add new am_conditional.
* Makefile.am: Skip doc subdir if BUILD_DOC is not set.
  • GnuPG-bug-id: T4263
  • Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>.