Update libsndfile to 1.0.29pre2+git

This pulls in MANY (over 890) changes compared to our
from our current 1.0.24 version.

lib-src/libsndfile/appveyor.yml

@@ -0,0 +1,23 @@
+version: 1.0.29pre1-{build}
+image: Visual Studio 2017
+configuration: Debug
+platform:
+  - Win32
+  - x64
+install:
+  - if %platform%==Win32 set VCPKG_TRIPLET=x86-windows
+  - if %platform%==x64 set VCPKG_TRIPLET=x64-windows
+  - vcpkg install libogg:%VCPKG_TRIPLET% libvorbis:%VCPKG_TRIPLET% libflac:%VCPKG_TRIPLET% sqlite3:%VCPKG_TRIPLET% opus:%VCPKG_TRIPLET%
+before_build:
+  - mkdir CMakeBuild
+  - cd CMakeBuild
+  - if %platform%==Win32 set CMAKE_GENERATOR=Visual Studio 15 2017
+  - if %platform%==x64 set CMAKE_GENERATOR=Visual Studio 15 2017 Win64
+  - cmake .. -G"%CMAKE_GENERATOR%" -DCMAKE_FIND_PACKAGE_PREFER_CONFIG=ON -DCMAKE_TOOLCHAIN_FILE=C:/Tools/vcpkg/scripts/buildsystems/vcpkg.cmake
+build:
+  project: c:\projects\libsndfile\CMakeBuild\sndfile.sln
+  verbosity: minimal
+test_script:
+  - ctest -V
+cache:
+  C:\Tools\vcpkg\Installed -> appveyor.yml