cmake: Silence clang/gcc deprecation warnings.
cmake: Silence clang/gcc deprecation warnings.

Parts of libwebm continue to use std::auto_ptr for support of older
compilers and target systems. Silence the warnings its use causes.

Change-Id: Iac8f35c9fdaaeb1cbb637d0777b46cdad54843ed

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm