internal openh264 subproject prevents gst-libav to build

When configuring gst-build, gst-libav is disabled with this message:

gst-libav: NO Dependencies must be external dependencies

After some further investigation, it appears to depend on the meson version. Indeed with 0.53.0, the project is enabled

With meson's master branch, FFMpeg fails with this message

||Run-time dependency opencv found: NO (tried pkgconfig and cmake)
||Run-time dependency opencv found: NO (tried pkgconfig and cmake)
||Dependency openh264 found: YES 1.8.0 (cached)
|
|subprojects/FFmpeg/meson.build:1595:10: Exception: Dependencies must be external dependencies

Subproject gst-libav is buildable: NO (disabling)