meson: Use string variables extensively

The strings holding the names used for libraries have also been
moved to different variables. This way they would be less error
as these variables can be reused easily and any typing error
would be quickly detected.

vapi/meson.build

@@ -1,5 +1,5 @@
 gnome.generate_vapi(
-  'libnm',
+  libnm_name,
   sources: libnm_gir[0],
   packages: 'gio-2.0',
   install: true
@@ -12,7 +12,7 @@ if enable_libnm_glib
   ]
 
   libnm_util_vapi = gnome.generate_vapi(
-    'libnm-util',
+    libnm_util_name,
     sources: libnm_util_gir[0],
     packages: packages,
     install: true
@@ -21,7 +21,7 @@ if enable_libnm_glib
   packages += libnm_util_vapi
 
   gnome.generate_vapi(
-    'libnm-glib',
+    libnm_glib_name,
     sources: libnm_glib_gir[0],
     packages: packages,
     install: true