[gtkmm] docs/reference/meson.build: Update for Doxygen >= 1.8.16

[Kjell Ahlstedt <kjellahl src gnome org>]

commit b1284d679dcc170d6c24ac56e57e18c19f0913f4
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Jun 15 16:05:30 2020 +0200

    docs/reference/meson.build: Update for Doxygen >= 1.8.16
    
    Doxygen 1.8.16 and later does not store tag file names in the html files.
    This requires changes in meson.build and in doc-install.pl (in mm-common).
    Otherwise references to other modules won't be updated in the html files
    when they are installed.

 docs/reference/meson.build | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
---
diff --git a/docs/reference/meson.build b/docs/reference/meson.build
index 241aa23f..ad35111e 100644
--- a/docs/reference/meson.build
+++ b/docs/reference/meson.build
@@ -28,11 +28,26 @@ foreach module : tag_file_modules
     elif htmlrefdir == ''
       htmlrefdir = htmlrefpub
     endif
+    if htmlrefpub.startswith('/')
+      htmlrefpub = 'file://' + htmlrefpub
+    endif
+    if htmlrefdir.startswith('/')
+      htmlrefdir = 'file://' + htmlrefdir
+    endif
     doxygen_tagfiles += ' "' + doxytagfile + '=' + htmlrefpub + '"'
+
     if not htmlrefdir.endswith('/')
       htmlrefdir += '/'
     endif
+    if not htmlrefpub.endswith('/')
+      htmlrefpub += '/'
+    endif
+    # Doxygen <= 1.8.15
     docinstall_flags += ['-l', doxytagfile.split('/')[-1] + '@' + htmlrefdir]
+    if htmlrefpub != htmlrefdir
+      # Doxygen >= 1.8.16
+      docinstall_flags += ['-l', 's@' + htmlrefpub + '@' + htmlrefdir]
+    endif
   endif
 endforeach