cmake: ignore deprecation notes for Secure Transport

The Secure Transport interface we're currently using has been deprecated
with macOS 10.15. As we're currently in code freeze, we cannot migrate
to newer interfaces. As such, let's disable deprecation warnings for
our "schannel.c" stream.

src/CMakeLists.txt

@@ -300,6 +300,10 @@ FILE(GLOB SRC_GIT2 *.c *.h
 	streams/*.c streams/*.h
 	transports/*.c transports/*.h
 	xdiff/*.c xdiff/*.h)
+IF(APPLE)
+	# The old Secure Transport API has been deprecated in macOS 10.15.
+	SET_SOURCE_FILES_PROPERTIES(streams/stransport.c PROPERTIES COMPILE_FLAGS -Wno-deprecated)
+ENDIF()
 
 # the xdiff dependency is not (yet) warning-free, disable warnings as
 # errors for the xdiff sources until we've sorted them out