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/FindFLAC.cmake

@@ -0,0 +1,67 @@
+# - Find FLAC
+# Find the native FLAC includes and libraries
+#
+#  FLAC_INCLUDE_DIRS - where to find FLAC headers.
+#  FLAC_LIBRARIES    - List of libraries when using libFLAC.
+#  FLAC_FOUND        - True if libFLAC found.
+#  FLAC_DEFINITIONS  - FLAC compile definitons 
+
+if (FLAC_INCLUDE_DIR)
+    # Already in cache, be silent
+    set (FLAC_FIND_QUIETLY TRUE)
+endif ()
+
+find_package (Ogg QUIET)
+
+find_package (PkgConfig QUIET)
+pkg_check_modules(PC_FLAC QUIET flac)
+
+set(FLAC_VERSION ${PC_FLAC_VERSION})
+
+find_path (FLAC_INCLUDE_DIR FLAC/stream_decoder.h
+	HINTS
+		${PC_FLAC_INCLUDEDIR}
+		${PC_FLAC_INCLUDE_DIRS}
+		${FLAC_ROOT}
+	)
+
+# MSVC built libraries can name them *_static, which is good as it
+# distinguishes import libraries from static libraries with the same extension.
+find_library (FLAC_LIBRARY
+	NAMES
+		FLAC
+		libFLAC
+		libFLAC_dynamic
+		libFLAC_static
+	HINTS
+		${PC_FLAC_LIBDIR}
+		${PC_FLAC_LIBRARY_DIRS}
+		${FLAC_ROOT}
+	)
+
+# Handle the QUIETLY and REQUIRED arguments and set FLAC_FOUND to TRUE if
+# all listed variables are TRUE.
+include (FindPackageHandleStandardArgs)
+find_package_handle_standard_args (FLAC
+	REQUIRED_VARS
+		FLAC_LIBRARY
+		FLAC_INCLUDE_DIR
+		OGG_FOUND
+	VERSION_VAR
+        FLAC_VERSION
+	)
+
+if (FLAC_FOUND)
+	set (FLAC_INCLUDE_DIRS ${FLAC_INCLUDE_DIR})
+	set (FLAC_LIBRARIES ${FLAC_LIBRARY} ${OGG_LIBRARIES})
+    if (NOT TARGET FLAC::FLAC)
+		add_library(FLAC::FLAC UNKNOWN IMPORTED)
+		set_target_properties(FLAC::FLAC PROPERTIES
+			INTERFACE_INCLUDE_DIRECTORIES "${FLAC_INCLUDE_DIR}"
+			IMPORTED_LOCATION "${FLAC_LIBRARY}"
+			INTERFACE_LINK_LIBRARIES Ogg::ogg
+			)
+	endif ()
+endif ()
+
+mark_as_advanced(FLAC_INCLUDE_DIR FLAC_LIBRARY)