meson: fix a few meson warnings

WARNING: The variable(s) 'DATADIR', 'LIBDIR' in the input file
  'subprojects/gst-devtools/validate/launcher/config.py.in' are not
  present in the given configuration data
WARNING: Passed invalid keyword argument "scanobj_args". This will
  become a hard error in the future.
WARNING: Keyword argument "install" defined multiple times. This
  will be a an error in future Meson releases.

validate/docs/validate/meson.build

@@ -24,7 +24,7 @@ if gtkdoc.found()
                 '--ignore-decorators=GST_EXPORT',
                 '--ignore-headers=gettext.h gst-validate-internal.h gst-validate-monitor.h gst-validate-bin-monitor.h gst-validate-element-monitor.h gst-validate-pad-monitor.h gst-validate-override.h gst-validate-override-registry.h gst-validate-utils.h gst-validate-media-info.h  gst-validate-report.h  media-descriptor.h media-descriptor-parser.h media-descriptor-writer.h gst-validate-i18n-lib.h'
             ],
-    scanobj_args : ['--type-init-func="gst_init(NULL,NULL)'],
+    scanobjs_args : ['--type-init-func="gst_init(NULL,NULL)"'],
     gobject_typesfile : types,
     dependencies : [validate_dep],
     content_files : ['gst-validate.xml', 'gst-validate-transcoding.xml', 'gst-validate-media-check.xml', 'gst-validate-launcher.xml', 'envvariables.xml', 'scenarios.xml'],

validate/launcher/meson.build

@@ -3,6 +3,9 @@ _launcherdir = get_option('libdir') + '/gst-validate-launcher/python/launcher/'
 launcher_configure = configuration_data()
 launcher_configure.set('GST_VALIDATE_TESTSUITE_VERSION', '"@0@"'.format(TESTSUITE_VERSION))
 launcher_configure.set('BUILDDIR', meson.build_root())
+launcher_configure.set('DATADIR', join_paths(get_option('prefix'), get_option('datadir')))
+launcher_configure.set('LIBDIR', join_paths(get_option('prefix'), get_option('libdir')))
+
 configure_file(input : 'config.py.in',
   output : 'config.py',
   install_dir: _launcherdir,

validate/plugins/gtk/meson.build

@@ -1,5 +1,4 @@
 shared_library('gstvalidategtk', 'gstvalidategtk.c',
-        install: true,
         include_directories : inc_dirs,
         dependencies : [gst_dep, glib_dep, gst_pbutils_dep, gtk_dep],
         c_args: ['-DHAVE_CONFIG_H'],