meson dependency subproject fallback

This patch helps to build wxrd without having the next version of xrdesktop, gxr and gulkan packaged.

All subprojects can be cloned in the wxrc subproject folder, as such:

wxrc
└── subprojects
    ├── gulkan
    ├── gxr
    ├── wlroots
    └── xrdesktop

Requires xrdesktop!36 (merged) and gxr!25 (merged)