CMake: disable deprecated documentation sync

The `-Wdocumentation-deprecated-sync` option will warn when there is a
doxygen `\deprecated` tag but there is no corresponding deprecation
attribute on the function.  We want to encourage users to not use
particular APIs by marking them deprecated in the documentation without
necessarily raising a compiler warning by marking an item as deprecated.

Author: ethomson

CMakeLists.txt

@@ -233,6 +233,7 @@ ELSE ()
 	ENABLE_WARNINGS(format)
 	ENABLE_WARNINGS(format-security)
 	ENABLE_WARNINGS(int-conversion)
+	DISABLE_WARNINGS(documentation-deprecated-sync)
 
 	IF (APPLE) # Apple deprecated OpenSSL
 	    DISABLE_WARNINGS(deprecated-declarations)