GoogleTest, starting with vesion 1.13.0, requires C++14 to build. This
patch enables C++14 if GoogleTest 1.13.0 or newer is detected when
compiling the tests with Meson, making it possible to use new GTest
versions.
You can find GoogleTest's release notes at
<https://github.com/google/googletest/releases/tag/v1.13.0>.