opencv: meson build fails on macOS

I got the following error when building on mac using gst-build. Add the c++11 flag to meson.build.

Environment

• macOS Catalina 10.15.7
• apple clang version 12.0.0 (clang-1200.0.32.29)
• gst-build v1.19.1
• opencv 4.5.0_5 (installed from brew)

Error Log

/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:194:59: warning: deleted function definitions are a C++11 extension [-Wc++11-extensions]
    _InputArray(const std::vector<std::vector<bool> >&) = delete;  // not supported
                                                          ^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:319:44: warning: deleted function definitions are a C++11 extension [-Wc++11-extensions]
    _OutputArray(std::vector<bool>& vec) = delete;  // not supported
                                           ^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:321:54: warning: deleted function definitions are a C++11 extension [-Wc++11-extensions]
    _OutputArray(std::vector<std::vector<bool> >&) = delete;  // not supported
                                                     ^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:393:49: warning: deleted function definitions are a C++11 extension [-Wc++11-extensions]
    _InputOutputArray(std::vector<bool>& vec) = delete;  // not supported
                                                ^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:1011:29: error: no template named 'initializer_list' in namespace 'std'
    explicit Mat(const std::initializer_list<_Tp> list);
                       ~~~~~^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:1010:37: warning: default template arguments for a function template are a C++11 extension [-Wc++11-extensions]
    template<typename _Tp, typename = typename std::enable_if<std::is_arithmetic<_Tp>::value>::type>
                                    ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:1015:52: error: no template named 'initializer_list' in namespace 'std'
    template<typename _Tp> explicit Mat(const std::initializer_list<int> sizes, const std::initializer_list<_Tp> list);
                                              ~~~~~^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:1015:92: error: no template named 'initializer_list' in namespace 'std'
    template<typename _Tp> explicit Mat(const std::initializer_list<int> sizes, const std::initializer_list<_Tp> list);
                                                                                      ~~~~~^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:2071:12: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
    Mat(Mat&& m);
           ^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:2072:25: warning: rvalue references are a C++11 extension [-Wc++11-extensions]
    Mat& operator = (Mat&& m);
                        ^
/usr/local/Cellar/opencv/4.5.0_5/include/opencv4/opencv2/core/mat.hpp:2226:15: error: no template named 'initializer_list' in namespace 'std'
    Mat_(std::initializer_list<_Tp> values);
         ~~~~~^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
35 warnings and 20 errors generated.
[598/3881] Compiling C object subprojects/gst-plugins-bad/gst-libs/gst/play/libgstplay-1.0.0.dylib.p/gstplay.c.o
ninja: build stopped: subcommand failed.