configure, meson, devel-docs: make gi-docgen into an auto option.

Also called "feature" option in meson, so that by default it depends on
auto-detection of the gi-docgen hence won't break the configuration when
the tool is missing (the feature is simply disabled).

Also move the program check into the root meson file, which is anyway
much better to have a better visibility of features, otherwise we'd end
up just having tests everywhere in any possible random directory of the
repo.

Finally add a line in the summary of the configuration step, displaying
the docs generation being enabled or disabled.

configure.ac

@@ -2752,13 +2752,21 @@ fi
 # Check for gi-docgen
 ########################################
 
-AC_ARG_ENABLE(gi-docgen, [  --enable-gi-docgen       build gi-docgen documentation (default=yes)], , enable_gi_docgen=yes)
+AC_ARG_ENABLE(gi-docgen, [  --enable-gi-docgen       build gi-docgen documentation (default=auto)], , enable_gi_docgen=auto)
 
-if test "x$enable_gi_docgen" = xyes; then
+if test "x$enable_gi_docgen" != xno; then
   AC_PATH_PROG(GI_DOCGEN, gi-docgen, no)
   if test "x$GI_DOCGEN" = xno; then
-    add_deps_error([gi-docgen], [Could not find gi-docgen in your PATH.])
+    if test "x$enable_gi_docgen" = xyes; then
+      add_deps_error([gi-docgen], [Could not find gi-docgen in your PATH.])
+    else
+      enable_gi_docgen="no (gi-docgen missing)"
+    fi
+  else
+    enable_gi_docgen="yes"
   fi
+else
+  enable_gi_docgen="no"
 fi
 
 AM_CONDITIONAL(ENABLE_GI_DOCGEN, test "x$enable_gi_docgen" = xyes)
@@ -3272,6 +3280,7 @@ Tests:
   Test desktop file          $have_desktop_file_validate
 
 Documentation:
+  libgimp API Reference:     $enable_gi_docgen
   GObject Introspection:     $enable_g_ir_doc
 
 Bug report URL: $with_bug_report_url

devel-docs/meson.build

@@ -16,8 +16,7 @@ mkdb_args_common = [
   '--name-space=gimp',
 ]
 
-
-if get_option('gi-docgen')
+if gi_docgen.found()
   subdir('reference')
 endif
 

devel-docs/reference/meson.build

@@ -1,4 +1,2 @@
-gi_docgen = find_program('gi-docgen')
-
 subdir('gimp')
 subdir('gimp-ui')

meson.build

@@ -1009,6 +1009,9 @@ desktop_validate    = find_program('desktop-file-validate', required: false)
 appstream_util = find_program('appstream-util', required: get_option('appdata-test'))
 
 # Check for doc generation tools
+
+gi_docgen = find_program('gi-docgen', required: get_option('gi-docgen'))
+
 if get_option('g-ir-doc')
   gir_doc_tool = find_program('g-ir-doc-tool', required: true)
   yelp_build   = find_program('yelp-build', required: true)
@@ -1828,6 +1831,7 @@ final_message = [
 '''  Test appdata         @0@'''.format(appstream_util.found()),
 '',
 '''Documentation:''',
+'''  libgimp API Reference: @0@'''.format(gi_docgen.found()),
 '''  GObject Introspection: @0@'''.format(get_option('g-ir-doc')),
 '',
 '''Bug report URL: @0@'''.format(bug_report_url),

meson_options.txt

@@ -51,7 +51,7 @@ option('xcursor',           type: 'feature', value: 'auto', description: 'Xcurso
 option('xpm',               type: 'feature', value: 'auto', description: 'XPM support')
 option('headless-tests',    type: 'feature', value: 'auto', description: 'Use xvfb-run/dbus-run-session for UI-dependent automatic tests')
 
-option('gi-docgen',         type: 'boolean', value: true,   description: 'Build developer documentation (uses gi-docgen)')
+option('gi-docgen',         type: 'feature', value: 'auto', description: 'Build developer documentation (uses gi-docgen)')
 option('g-ir-doc',          type: 'boolean', value: false,  description: 'Build developer documentation for introspected API')
 
 option('linux-input',       type: 'feature', value: 'auto', description: 'Linux input event controller module')