soundtouch: Fix build failure with Apple Clang caused by missing cpp_std
Hi,
This MR is sent in preparation for the update of Soundtouch to 2.3.2 and its mesonification.
When I was testing the Cerbero build, I noticed that it failed build on macOS specifically. I tracked this down to the fact that Apple Clang sets C++98 by default.
In this MR, I'm applying C++14 to account for Meson's warning about lack of support/fallback for cpp_std=c++11
under MSVC.