build: no more make recursion for 'doc' subdir

build: no more make recursion for 'doc' subdir

* .gitignore: Adjust.
* Makefile.am ($(srcdir)/doc/local.mk): New include.
(SUBDIRS): Drop 'doc'.
(AM_MAKEINFOFLAGS): Rename ...
(custom_MAKEINFOFLAGS): ... like this, to avoid conflicting with
the AM_MAKEINFOFLAGS defined in the included 'doc/local.mk'
($(srcdir)/INSTALL): Adjust recipe.
* doc/Makefile.am: Rename ...
* doc/local.mk: ... like this, and adjust.
* configure.ac (AC_CONFIG_FILES): Drop 'doc/Makefile'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>