[gxml] docs: disabling --driver for vala >= 42

[Daniel Espinosa Ortiz <despinosa src gnome org>]

commit 9910fcbb2265e9c93c76f7f3559fdc4443e118bb
Author: Daniel Espinosa <esodan gmail com>
Date:   Mon Jul 8 10:26:32 2019 -0500

    docs: disabling --driver for vala >= 42
    
    Fix issue #25

 docs/meson.build | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)
---
diff --git a/docs/meson.build b/docs/meson.build
index c20fa91..aad3442 100644
--- a/docs/meson.build
+++ b/docs/meson.build
@@ -10,11 +10,18 @@ vapidirgxml = '--vapidir='+join_paths (meson.current_source_dir (),'gxml')
 vapidirvapi = '--vapidir='+join_paths (meson.current_source_dir (),'vapi')
 docsdir = join_paths (get_option ('datadir'), 'devhelp','books')
 
+driver_ops = []
+if valacapi.stdout().version_compare('>=0.42')
+       message ('Vala is >= 0.42: Disabling --driver for documentation')
+else
+       driver_ops += driver
+endif
+
 custom_target ('valadocs',
                input : sources,
                output : outdir,
                command : [valadoc,
-                       driver,
+                       driver_ops,
                        '--doclet=devhelp',
                        '--force',
                        pkgname,