Alex James

Meson unconditionally uses headers from the wrapped vapoursynth subproject when the vapoursynth option is enabled. Update meson.build to support using system-provided vapoursynth headers.

Meson currently builds gtest from a subproject before running the tests. Update tests/meson.build to support using a system-provided version of gtest and gmock when they are present.

The `std::basic_string` template specialization for `int16_t` was [deprecated in libcxx 18](https://reviews.llvm.org/D138307) and [removed in libcxx 19](https://reviews.llvm.org/D157058), which broke compilation of the iconv test: ``` FAILED: tests/gtest-main.p/tests_iconv.cpp.o clang++ -Itests/gtest-main.p -Itests -I../tests...