meson: Define cpp_std in the top-level project()

meson: Define cpp_std in the top-level project()

Due to a bug, meson ignores ${lang}_std settings in default_options
for subprojects: https://github.com/mesonbuild/meson/issues/1889

This causes build failures when a subproject requires c++11 or c++14,
etc. Compilers that support those cpp_stds are very common, and all
the toolchains that we support include c++ compilers, so we can
add cpp_std=c++14 to the top-level.

This fixes the webrtc-audio-processing build on Linux, and harfbuzz on
macOS.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1133>