Build gst-libav if libav is on the system

meson.build

@@ -15,6 +15,29 @@ subprojects = [
     'gst-editing-services'
 ]
 
+# FIXME Remove that check once we have ffmpeg as a gst-libav subproject
+libavfilter_dep = dependency('libavfilter', version: '>= 6.47.100', required: false)
+gst_libav = []
+if libavfilter_dep.found()
+    cc = meson.get_compiler('c')
+    check_ffmpeg_src = '''#include <libavcodec/avcodec.h>
+    #if LIBAVCODEC_VERSION_MICRO >= 100
+    /* FFmpeg uses 100+ as its micro version */
+    #else
+    #error libav provider should be FFmpeg
+    #endif'''
+    if cc.compiles(check_ffmpeg_src, name : 'whether libav is provided by FFmpeg')
+        gst_libav = ['gst-libav']
+    endif
+endif
+
+if gst_libav.length() == 0
+    message('WARNING: gst-libav not built as ffmpeg >= n3.1.2 not found on the system')
+else
+    subprojects += gst_libav
+endif
+
+
 foreach subproj: subprojects
     subproject(subproj, version: gst_version)
 endforeach

subprojects/gst-libav.wrap

@@ -0,0 +1,4 @@
+[wrap-git]
+directory=gst-libav
+url=git://anongit.freedesktop.org/gstreamer/gst-libav
+revision=master