gst-plugins-bad: meson build fails on ubuntu 16.04: ext/webrtcdsp/meson.build forgot to set c++11 mode

Building gst-plugins-bad 1.15.2 failed for me just now on Ubuntu 16.04.

ext/webrtcdsp/Makefile.am sets c++11 mode, but ext/webrtcdsp/meson.build doesn't. Hilarity ensues:

+ meson --prefix=/opt/oblong/deps-64-13 -Dexamples=disabled -Dnls=disabled -Dnvdec=disabled -Dnvenc=disabled -Dopencv=disabled -Dopenexr=disabled -Drsvg=disabled -Dwayland=disabled . meson-build.tmp
The Meson build system
Version: 0.50.0
...
Native C++ compiler: c++ (gcc 5.4.0 "c++ (Ubuntu 5.4.0-6ubuntu1~16.04.11) 5.4.0 20160609")
...
FAILED: c++ -Iext/webrtcdsp/02f25f2@@gstwebrtcdsp@sha -Iext/webrtcdsp -I../ext/webrtcdsp -I. -I../ -Igst-libs -I../gst-libs -I/opt/oblong/deps-64-13/include/glib-2.0 -I/opt/oblong/deps-64-13/lib/glib-2.0/include -I/opt/oblong/deps-64-13/include/gstreamer-1.0 -I/opt/oblong/deps-64-13/include/orc-0.4 -I/opt/oblong/deps-64-13/include/webrtc_audio_processing -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wnon-virtual-dtor -O2 -g -Wno-non-virtual-dtor -fno-strict-aliasing -Wformat-nonliteral -Wmissing-declarations -Wredundant-decls -Wwrite-strings -Wformat -Wformat-security -Winit-self -Wmissing-include-dirs -Waddress -Wno-multichar -Wvla -Wpointer-arith -fPIC -DWEBRTC_AUDIO_PROCESSING_ONLY_BUILD -DWEBRTC_POSIX -pthread -DHAVE_CONFIG_H -MD -MQ 'ext/webrtcdsp/02f25f2@@gstwebrtcdsp@sha/gstwebrtcdsp.cpp.o' -MF 'ext/webrtcdsp/02f25f2@@gstwebrtcdsp@sha/gstwebrtcdsp.cpp.o.d' -o 'ext/webrtcdsp/02f25f2@@gstwebrtcdsp@sha/gstwebrtcdsp.cpp.o' -c ../ext/webrtcdsp/gstwebrtcdsp.cpp
...
/opt/oblong/deps-64-13/include/webrtc_audio_processing/webrtc/base/maybe.h:34:7: warning: override controls (override/final) only available with -std=c++11 or -std=gnu++11
 class Maybe final {
       ^
/opt/oblong/deps-64-13/include/webrtc_audio_processing/webrtc/base/maybe.h:41:19: error: expected ‘,’ or ‘...’ before ‘&&’ token
   explicit Maybe(T&& val) : value_(static_cast<T&&>(val)), has_value_(true) {}
                   ^

Fix should be a one-liner in ext/webrtcdsp/meson.build, testing now.