gnome.gtkdoc: Handle absolute install_dirs correctly

Must prepend DESTDIR in case it's absolute. Also document that by
default it is relative to the gtk-doc html directory.

docs/markdown/Gnome-module.md

@@ -136,14 +136,14 @@ Note that very old versions of yelp may not support symlinked media; At least 3.
 
 ### gnome.gtkdoc()
 
-Compiles and installs gtkdoc documentation. Takes one positional arguments; The name of the module.
+Compiles and installs gtkdoc documentation into `prefix/share/gtk-doc/html`. Takes one positional argument: The name of the module.
 
 * `main_xml`: specifies the main XML file
 * `main_sgml`: equal to `main_xml`
 * `src_dir`: include_directories to include
 * `dependencies`: a list of dependencies
 * `install`: if true, installs the generated docs
-* `install_dir`: the directory to install the generated docs
+* `install_dir`: the directory to install the generated docs relative to the gtk-doc html dir or an absolute path (default: module name)
 * `scan_args`: a list of arguments to pass to `gtkdoc-scan`
 * `scanobjs_args`: a list of arguments to pass to `gtkdoc-scangobj`
 * `gobject_typesfile`: a list of type files

mesonbuild/scripts/gtkdochelper.py

@@ -218,12 +218,14 @@ def run(args):
         options.mode)
 
     if 'MESON_INSTALL_PREFIX' in os.environ:
-        install_dir = options.install_dir if options.install_dir else options.modulename
         destdir = os.environ.get('DESTDIR', '')
-        installdir = destdir_join(destdir, os.environ['MESON_INSTALL_PREFIX'])
+        install_prefix = destdir_join(destdir, os.environ['MESON_INSTALL_PREFIX'])
+        install_dir = options.install_dir if options.install_dir else options.modulename
+        if os.path.isabs(install_dir):
+            install_dir = destdir_join(destdir, install_dir)
         install_gtkdoc(options.builddir,
                        options.subdir,
-                       installdir,
+                       install_prefix,
                        'share/gtk-doc/html',
                        install_dir)
     return 0