build: add initial support for meson build system

meson is a build system focused on speed an ease of use, which
helps speeding up the software development. This patch adds meson
support along autotools.

[thaller@redhat.com: rebased patch and adjusted for iwd support]

https://mail.gnome.org/archives/networkmanager-list/2017-December/msg00022.html

vapi/meson.build

@@ -0,0 +1,29 @@
+gnome.generate_vapi(
+  'libnm',
+  sources: libnm_gir[0],
+  packages: 'gio-2.0',
+  install: true
+)
+
+if enable_libnm_glib
+  packages = [
+    'dbus-glib-1',
+    'gio-2.0'
+  ]
+
+  libnm_util_vapi = gnome.generate_vapi(
+    'libnm-util',
+    sources: libnm_util_gir[0],
+    packages: packages,
+    install: true
+  )
+
+  packages += libnm_util_vapi
+
+  gnome.generate_vapi(
+    'libnm-glib',
+    sources: libnm_glib_gir[0],
+    packages: packages,
+    install: true
+  )
+endif