meson: build oss4 plugin

https://bugzilla.gnome.org/show_bug.cgi?id=784134

sys/meson.build

@@ -1,10 +1,10 @@
 subdir('directsound')
 subdir('oss')
+subdir('oss4')
 subdir('v4l2')
 subdir('ximage')
 
 # FIXME: Implement these
-#subdir('oss4')
 #subdir('osxaudio')
 #subdir('osxvideo')
 #subdir('sunaudio')

sys/oss4/meson.build

@@ -0,0 +1,23 @@
+have_oss4 = true
+message('Checking headers needed for Open Sound System 4 plugin...')
+foreach hdr : ['fcntl.h', 'sys/ioctl.h', 'sys/stat.h', 'sys/types.h']
+  if have_oss4
+    if not cc.has_header(hdr)
+      have_oss4 = false
+    endif
+  endif
+endforeach
+
+if have_oss4
+  message('Required headers found, building Open Sound System 4 plugin.')
+
+  library('gstoss4',
+    'oss4-audio.c', 'oss4-property-probe.c', 'oss4-sink.c', 'oss4-source.c',
+    c_args : gst_plugins_good_args,
+    include_directories : [configinc, libsinc],
+    dependencies : [gstaudio_dep, gstbase_dep],
+    install : true,
+    install_dir : plugins_install_dir)
+else
+  message('Not building Open Sound System 4 plugin, required headers not found.')
+endif