Vala : Allow ignoring generated files

[BZHDeveloper1986]

Describe the bug
In a project combining Vala and C, generated vapi is included as source.
If the C fix brings new compat functions, example, when an interface is not compatible with Vala (Hello GStreamer !), and that the VAPI patch is provided in a custom directory, vala_args '--vapidir' are ignored

To Reproduce
needed : valac, GStreamer plugins base library, GLib
code : gst-utils.tar.gz

Expected behavior
Use of local vapi file, with an on/off option for generated vapi

system parameters

• Native build
• Ubuntu 24.04
• Python 3.13.2
• Meson 1.6.1
• Ninja 1.12.1