Documentation: Add predefined macros from config.h to Doxyfile

libcamera creates a config.h file with predefined macros, and instructs
the compiler to include it implicitly with the -include argument.
Doxygen has no support for implicit inclusion of headers, but has a
PREDEFINED configuration option for its preprocessor that lists
predefined macros. Populate it with the values from the config_h
configuration data object that is used for generate the config.h file,
to ensure that documentation matches the configuration options libcamera
has been built with.

Bump the minimum meson version to 0.57 to use cfg_data.keys().

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

Documentation/meson.build

@@ -16,6 +16,13 @@ if doxygen.found() and dot.found()
     cdata.set('TOP_BUILDDIR', meson.project_build_root())
     cdata.set('OUTPUT_DIR', meson.current_build_dir())
 
+    doxygen_predefined = []
+    foreach key : config_h.keys()
+        doxygen_predefined += '@0@=@1@'.format(key, config_h.get(key))
+    endforeach
+
+    cdata.set('PREDEFINED', ' \\\n\t\t\t '.join(doxygen_predefined))
+
     doxyfile = configure_file(input : 'Doxyfile.in',
                               output : 'Doxyfile',
                               configuration : cdata)