configure: automatically parse command line for meson -D options

Right now meson_options.txt lists about 90 options.  Each option
needs code in configure to parse it and pass the option down to Meson as
a -D command-line argument; in addition the default must be duplicated
between configure and meson_options.txt.  This series tries to remove
the code duplication by generating the case statement for those --enable
and --disable options, as well as the corresponding help text.

About 80% of the options can be handled completely by the new mechanism.
Eight meson options are not of the --enable/--disable kind.  Six more need
to be parsed in configure for various reasons documented in the patch,
but they still have their help automatically generated.

The advantages are:

- less code in configure

- parsing and help is more consistent (for example --enable-blobs was
  not supported)

- options are described entirely in one place, meson_options.txt.
  This make it more attractive to use Meson options instead of
  hand-crafted configure options and config-host.mak

A few options change name: --enable-tcmalloc and --enable-jemalloc
become --enable-malloc={tcmalloc,jemalloc}; --disable-blobs becomes
--disable-install-blobs; --enable-trace-backend becomes
--enable-trace-backends.  However, the old names are allowed
for backwards compatibility.

Message-Id: <20211007130829.632254-19-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/devel/build-system.rst

@@ -42,73 +42,21 @@ perform a build:
      ../configure
      make
 
-For now, checks on the compilation environment are found in configure
-rather than meson.build, though this is expected to change.  The command
-line is parsed in the configure script and, whenever needed, converted
-into the appropriate options to Meson.
-
-New checks should be added to Meson, which usually comprises the
-following tasks:
-
- - Add a Meson build option to meson_options.txt.
-
- - Add support to the command line arg parser to handle any new
-   ``--enable-XXX``/``--disable-XXX`` flags required by the feature.
-
- - Add information to the help output message to report on the new
-   feature flag.
-
- - Add code to perform the actual feature check.
-
- - Add code to include the feature status in ``config-host.h``
-
- - Add code to print out the feature status in the configure summary
-   upon completion.
-
-
-Taking the probe for SDL2_Image as an example, we have the following pieces
-in configure::
-
-  # Initial variable state
-  sdl_image=auto
-
-  ..snip..
-
-  # Configure flag processing
-  --disable-sdl-image) sdl_image=disabled
-  ;;
-  --enable-sdl-image) sdl_image=enabled
-  ;;
-
-  ..snip..
-
-  # Help output feature message
-  sdl-image         SDL Image support for icons
-
-  ..snip..
-
-  # Meson invocation
-  -Dsdl_image=$sdl_image
-
-In meson_options.txt::
-
-  option('sdl', type : 'feature', value : 'auto',
-         description: 'SDL Image support for icons')
-
-In meson.build::
-
-  # Detect dependency
-  sdl_image = dependency('SDL2_image', required: get_option('sdl_image'),
-                         method: 'pkg-config',
-                         kwargs: static_kwargs)
-
-  # Create config-host.h (if applicable)
-  config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
-
-  # Summary
-  summary_info += {'SDL image support': sdl_image.found()}
+The configure script automatically recognizes
+command line options for which a same-named Meson option exists;
+dashes in the command line are replaced with underscores.
 
+Many checks on the compilation environment are still found in configure
+rather than `meson.build`, but new checks should be added directly to
+`meson.build`.
 
+Patches are also welcome to move existing checks from the configure
+phase to `meson.build`.  When doing so, ensure that `meson.build` does
+not use anymore the keys that you have removed from `config-host.mak`.
+Typically these will be replaced in `meson.build` by boolean variables,
+``get_option('optname')`` invocations, or `dep.found()` expressions.
+In general, the remaining checks have little or no interdependencies,
+so they can be moved one by one.
 
 Helper functions
 ----------------
@@ -335,6 +283,60 @@ new target, or enabling new devices or hardware for a particular
 system/userspace emulation target
 
 
+Adding checks
+-------------
+
+New checks should be added to Meson.  Compiler checks can be as simple as
+the following::
+
+  config_host_data.set('HAVE_BTRFS_H', cc.has_header('linux/btrfs.h'))
+
+A more complex task such as adding a new dependency usually
+comprises the following tasks:
+
+ - Add a Meson build option to meson_options.txt.
+
+ - Add code to perform the actual feature check.
+
+ - Add code to include the feature status in `config-host.h`
+
+ - Add code to print out the feature status in the configure summary
+   upon completion.
+
+Taking the probe for SDL2_Image as an example, we have the following
+in ``meson_options.txt``::
+
+  option('sdl_image', type : 'feature', value : 'auto',
+         description: 'SDL Image support for icons')
+
+Unless the option was given a non-``auto`` value (on the configure
+command line), the detection code must be performed only if the
+dependency will be used::
+
+  sdl_image = not_found
+  if not get_option('sdl_image').auto() or have_system
+    sdl_image = dependency('SDL2_image', required: get_option('sdl_image'),
+                           method: 'pkg-config',
+                           static: enable_static)
+  endif
+
+This avoids warnings on static builds of user-mode emulators, for example.
+Most of the libraries used by system-mode emulators are not available for
+static linking.
+
+The other supporting code is generally simple::
+
+  # Create config-host.h (if applicable)
+  config_host_data.set('CONFIG_SDL_IMAGE', sdl_image.found())
+
+  # Summary
+  summary_info += {'SDL image support': sdl_image.found()}
+
+For the configure script to parse the new option, the
+``scripts/meson-buildoptions.sh`` file must be up-to-date; ``make
+update-buildoptions`` (or just `make`) will take care of updating it.
+
+
 Support scripts
 ---------------