Now that all warnings are taken care of, add warning flag -Wshadow to
configure.

Note: Enabling this flag for C++ generates some useless warnings about
some function parameters shadowing class member function names. So, only
enabling this warning for C code.

Cherry-picked from aomedia/master: b96cbc44

Change-Id: I3922dea2e6976b16519c4aa4d1bd395c198134f1