configure: Improve help-options handling

This patch includes:

* A README.help.md as a general documentation for the different
  help types, the LO code and help-related build options.
* Adds --disable-xmlhelp for removing the xmlhelp support from the
  build. Disable for iOS, Android and Emscripten. This was partly
  included in HAVE_FEATURE_DESKTOP before.
* Rename WITH_HELP define to WITH_HELPPACKS, which reflects the
  actual usage AFAIK.
* Depend --with-omindex on --with-help=online and don't override
  the --with-help setting. Error out on conflicting options.
* Depend --with-helppack-integration on build help variants, which
  actually result in help packs.

Kind of reverts commit 2c38ea6d16
("Building without --with-help is not supposed to disable help
functionality").

Change-Id: Ie4cb73905b3ed94e991d9f1bd75cfbd6de9da385
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/116222
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>

RepositoryModule_host.mk

@@ -60,7 +60,7 @@ $(eval $(call gb_Module_add_moduledirs,libreoffice,\
 	formula \
 	$(call gb_Helper_optional,DESKTOP,fpicker) \
 	framework \
-	$(call gb_Helper_optional,DESKTOP,helpcompiler) \
+    $(call gb_Helper_optionals_or,HELPTOOLS XMLHELP,helpcompiler) \
 	$(call gb_Helper_optional,HELP,helpcontent2) \
 	hwpfilter \
 	i18nlangtag \
@@ -153,7 +153,7 @@ $(eval $(call gb_Module_add_moduledirs,libreoffice,\
 	writerfilter \
 	writerperfect \
 	xmerge \
-	$(call gb_Helper_optional,DESKTOP,xmlhelp) \
+    $(call gb_Helper_optional,XMLHELP,xmlhelp) \
 	xmloff \
 	xmlreader \
 	xmlscript \