GL plugin build failure using meson in 1.14

This happens both in a Flatpak with the 18.08 fd.o runtime, and Fedora 29.

$ git clone git@gitlab.freedesktop.org:gstreamer/gst-plugins-bad.git
$ cd gst-plugins-bad
$ git checkout -b 1.14 origin/1.14
$ meson _build
$ cd _build
$ ninja
FAILED: ext/gl/libgstopenglmixers.so 
cc  -o ext/gl/libgstopenglmixers.so 'ext/gl/35d3624@@gstopenglmixers@sha/gstopengl.c.o' 'ext/gl/35d3624@@gstopenglmixers@sha/gstglbasemixer.c.o' 'ext/gl/35d3624@@gstopenglmixers@sha/gstglmixerbin.c.o' 'ext/gl/35d3624@@gstopenglmixers@sha/gstglmixer.c.o' 'ext/gl/35d3624@@gstopenglmixers@sha/gstglvideomixer.c.o' 'ext/gl/35d3624@@gstopenglmixers@sha/gstglstereomix.c.o' 'ext/gl/35d3624@@gstopenglmixers@sha/gstglutils.c.o' -Wl,--no-undefined -Wl,--as-needed -shared -fPIC -Wl,--start-group -Wl,-soname,libgstopenglmixers.so gst-libs/gst/video/libgstbadvideo-1.0.so.0.1404.0 /usr/lib64/libgstvideo-1.0.so /usr/lib64/libgstbase-1.0.so /usr/lib64/libgstreamer-1.0.so /usr/lib64/libgobject-2.0.so /usr/lib64/libglib-2.0.so /usr/lib64/libgstgl-1.0.so /usr/lib64/libgstcontroller-1.0.so -lm /usr/lib64/libX11.so -Wl,--end-group '-Wl,-rpath,$ORIGIN/../../gst-libs/gst/video' -Wl,-rpath-link,/tmp/gst-plugins-bad/_build/gst-libs/gst/video  
/usr/bin/ld: ext/gl/35d3624@@gstopenglmixers@sha/gstopengl.c.o: in function `plugin_init':
/tmp/gst-plugins-bad/_build/../ext/gl/gstopengl.c:99: undefined reference to `gst_gl_mosaic_get_type'
collect2: error: ld returned 1 exit status
[486/627] Compiling C object 'ext/hls/7059488@@gsthls@sha/gsthlsdemux.c.o'.
ninja: build stopped: subcommand failed.