Backport "Dependencies must be external dependencies error" into 4.1.5

commit 5da8bab7:

meson: Don't use internal dep during check
openh264 can be found internally and shoud not be enable in
FFMpeg. Use only external dependency.

Fixes "Dependencies must be external dependencies" meson error.

https://gitlab.freedesktop.org/gstreamer/gst-build/-/issues/87
https://gitlab.freedesktop.org/gstreamer/meson-ports/ffmpeg/-/merge_requests/31