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 2c38ea6d16b910294220cefaf8ae6a0683e6405a
("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>

1 files changed, 59 insertions, 0 deletions

diff --git a/README.help.md b/README.help.md
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index 000000000000..3c923bd7ef89
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+# LibreOffice help build
+
+LibreOffice can build three kinds of help output from helpcontent2:
+
+* XML (local, bundeled with LibreOffice < 6.0)
+* HTML (local, bundeled with LibreOffice >= 6.0)
+* HTML (online)
+
+Both local help variants can be included in help-packs, which might be bundeled
+with the installer or provided as separate packages. They are supposed to be
+installed by a user to provide localized help. Both HTML-based help types are
+displayed in a browser, while the XML help uses an internal viewer component.
+
+The XML help is the only one supported in extensions.
+
+## configure options
+
+--with-help: selects the help type to build
+--with-omindex: prepares the online help for a Xapian Omega based search index
+--disable-xmlhelp: removes support for the XML help
+--with-helppack-integration: includes the help-packs in the installer
+
+Also see `configure --help`.
+
+## XML help, bundeled
+
+This type was bundeled per-default until LibreOffice 6.0.
+
+This help output is generated by using two tools: HelpIndexer and HelpLinker.
+It uses an internal viewer component. Extensions still use this help type to
+ship their help. This output is originally based on JavaDoc.
+
+The main support for this help type is in the following directories:
+
+* helpcompiler: tooling and support library
+* xmlhelp: the viewer component
+* desktop/source/deployment/registry/help: extensions / help-pack support
+
+Most code is "marked" by HAVE_FEATURE_XMLHELP precompiler blocks or conditioned
+by the XMLHELP flag in the $BUILD_TYPE variable.
+
+Interestingly the HelpIndexer and HelpLinker are used by gbuild to build the
+help inside bundeled extensions, but both are not part of the LibreOffice SDK.
+
+## HTML help, bundeled
+
+This type is bundeled per-default since LibreOffice 6.0 and displayed in a
+browser.
+
+## HTML help, online
+
+This help is the external one, currently provided by help.libreoffice.org. It
+differs from the bundeled HTML help in three main aspects:
+
+* has a language chooser widget
+* provides an additional Xapian Omega, CGI- / server-based search index
+* includes some additional multimedia assets
+
+It is opened in a browser.