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/cmake/FindOpus.cmake

@@ -0,0 +1,67 @@
+# - Find opus
+# Find the native opus includes and libraries
+#
+#  OPUS_INCLUDE_DIRS - where to find opus.h, etc.
+#  OPUS_LIBRARIES    - List of libraries when using opus.
+#  OPUS_FOUND        - True if Opus found.
+
+if (OPUS_INCLUDE_DIR)
+    # Already in cache, be silent
+    set(OPUS_FIND_QUIETLY TRUE)
+endif ()
+
+find_package (Ogg QUIET)
+
+find_package (PkgConfig QUIET)
+pkg_check_modules(PC_OPUS QUIET opus>=1.1)
+
+set (OPUS_VERSION ${PC_OPUS_VERSION})
+
+find_path (OPUS_INCLUDE_DIR opus/opus.h
+	HINTS
+		${PC_OPUS_INCLUDEDIR}
+		${PC_OPUS_INCLUDE_DIRS}
+		${OPUS_ROOT}
+	)
+
+# MSVC built opus may be named opus_static.
+# The provided project files name the library with the lib prefix.
+
+find_library (OPUS_LIBRARY
+	NAMES
+		opus
+		opus_static
+		libopus
+		libopus_static
+	HINTS
+		${PC_OPUS_LIBDIR}
+		${PC_OPUS_LIBRARY_DIRS}
+		${OPUS_ROOT}
+	)
+
+# Handle the QUIETLY and REQUIRED arguments and set OPUS_FOUND
+# to TRUE if all listed variables are TRUE.
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args (Opus
+	REQUIRED_VARS
+		OPUS_LIBRARY
+		OPUS_INCLUDE_DIR
+		OGG_FOUND
+	VERSION_VAR
+		OPUS_VERSION
+	)
+
+if (OPUS_FOUND)
+	set (OPUS_LIBRARIES ${OPUS_LIBRARY})
+	set (OPUS_INCLUDE_DIRS ${OPUS_INCLUDE_DIR})
+
+	if (NOT TARGET Opus::opus)
+		add_library (Opus::opus UNKNOWN IMPORTED)
+		set_target_properties (Opus::opus PROPERTIES
+			INTERFACE_INCLUDE_DIRECTORIES "${OPUS_INCLUDE_DIRS}"
+			IMPORTED_LOCATION "${OPUS_LIBRARIES}"
+		)
+	endif ()
+endif ()
+
+mark_as_advanced(OPUS_INCLUDE_DIR OPUS_LIBRARY)