allow customization of the "doc" target to skip certain types of documentation

* doc/Makefile: read targets to make for "doc" from $(ORG_MAKE_DOC),
  defaults to "info html pdf"; can be overridden by the user in
  local.mk.

* targets.mk: read targets to make for "doc" and "docs" from
  $(ORG_MAKE_DOC), defaults to "info html pdf"; can be overridden by
  the user in local.mk.

* default.mk: document ORG_MAKE_DOC and provide a (commented) example
  of how to have make only produce info documentation.

default.mk

@@ -4,6 +4,9 @@
 ##  CHECK AND ADAPT THE FOLLOWING DEFINITIONS
 ##----------------------------------------------------------------------
 
+# Override default target if desired or define your own default target
+# oldorg:	# have plain "make" do the same things the old Makefile did
+
 # Name of your emacs binary
 EMACS	= emacs
 
@@ -19,6 +22,9 @@ datadir = $(prefix)/emacs/etc/org
 # Where info files go.
 infodir = $(prefix)/info
 
+# Define if you only need info documentation, the default includes html and pdf
+# ORG_MAKE_DOC = info # html pdf
+
 # Where to create temporary files for the testsuite
 TMPDIR ?= /tmp
 testdir = $(TMPDIR)/tmp-orgtest

doc/Makefile

@@ -6,7 +6,7 @@ endif
 		clean cleanall clean-install \
 		org-version.inc
 
-all:		info html pdf card
+all:		$(ORG_MAKE_DOC)
 
 info:		org
 

targets.mk

@@ -6,6 +6,7 @@ DISTFILES_extra=  Makefile request-assign-future.txt contrib etc
 LISPDIRS      = lisp
 SUBDIRS       = doc etc $(LISPDIRS)
 INSTSUB       = $(SUBDIRS:%=install-%)
+ORG_MAKE_DOC ?= info html pdf
 
 ifneq ($(wildcard .git),)
   GITVERSION ?= $(shell git describe --abbrev=6 HEAD)
@@ -62,7 +63,7 @@ install:	$(INSTSUB)
 
 install-info:	install-doc
 
-doc docs:	info html pdf card
+doc docs:	$(ORG_MAKE_DOC)
 
 info html pdf card:
 	$(MAKE) -C doc $@