doc: Check for 'dot' and add the relevant configuration

We don't really use it, right now, but it may come in handy later, and
it doesn't cost us anything, since the whole thing is optional anyway.

doc/Doxyfile.in

@@ -194,13 +194,48 @@ INCLUDE_PATH           = "@top_srcdir@/include" \
                          "@top_builddir@/include"
 INCLUDE_FILE_PATTERNS  = *.h
 PREDEFINED             = DOXYGEN_SHOULD_SKIP_THIS \
-                         "PUBLIC=" \
-                         "EPOXY_PUBLIC=" \
-                         "EPOXY_IMPORTEXPORT="
+                         "EPOXY_BEGIN_DECLS=" \
+                         "EPOXY_END_DECLS=" \
+                         "EPOXY_PUBLIC="
 EXPAND_AS_DEFINED      =
 SKIP_FUNCTION_MACROS   = YES
 
-CLASS_DIAGRAMS         = NO
 ALLEXTERNALS           = NO
 EXTERNAL_GROUPS        = NO
 EXTERNAL_PAGES         = NO
+
+HAVE_DOT               = @HAVE_DOT@
+CLASS_DIAGRAMS         = NO
+MSCGEN_PATH            =
+DIA_PATH               =
+HIDE_UNDOC_RELATIONS   = NO
+DOT_NUM_THREADS        = 0
+DOT_FONTNAME           = Sans
+DOT_FONTSIZE           = 10
+DOT_FONTPATH           =
+CLASS_GRAPH            = NO
+COLLABORATION_GRAPH    = YES
+GROUP_GRAPHS           = YES
+UML_LOOK               = NO
+UML_LIMIT_NUM_FIELDS   = 10
+TEMPLATE_RELATIONS     = NO
+INCLUDE_GRAPH          = NO
+INCLUDED_BY_GRAPH      = NO
+CALL_GRAPH             = NO
+CALLER_GRAPH           = NO
+GRAPHICAL_HIERARCHY    = YES
+DIRECTORY_GRAPH        = YES
+DOT_IMAGE_FORMAT       = png
+INTERACTIVE_SVG        = NO
+DOT_PATH               =
+DOTFILE_DIRS           =
+MSCFILE_DIRS           =
+DIAFILE_DIRS           =
+PLANTUML_JAR_PATH      =
+PLANTUML_INCLUDE_PATH  =
+DOT_GRAPH_MAX_NODES    = 50
+MAX_DOT_GRAPH_DEPTH    = 0
+DOT_TRANSPARENT        = NO
+DOT_MULTI_TARGETS      = YES
+GENERATE_LEGEND        = YES
+DOT_CLEANUP            = YES

doc/meson.build

@@ -4,6 +4,12 @@ doxyfile_conf.set('PACKAGE_VERSION', meson.project_version())
 doxyfile_conf.set('top_srcdir', meson.source_root())
 doxyfile_conf.set('top_builddir', meson.build_root())
 
+if find_program('dot', required: false).found()
+  doxyfile_conf.set('HAVE_DOT', 'YES')
+else
+  doxyfile_conf.set('HAVE_DOT', 'NO')
+endif
+
 doxyfile = configure_file(input: 'Doxyfile.in',
                           output: 'Doxyfile',
                           configuration: doxyfile_conf,