meson: Make the glvnd vendor name configurable

Leave the default as "mesa", but this enables parallel Mesa installs.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7674>

meson_options.txt

@@ -289,6 +289,12 @@ option(
   value : 'auto',
   description : 'Build support for the Microsoft CLC to DXIL compiler'
 )
+option(
+  'glvnd-vendor-name',
+  type : 'string',
+  value : 'mesa',
+  description : 'Vendor name string to use for glvnd libraries'
+)
 option(
    'glx-read-only-text',
    type : 'boolean',