PR c/53066 Wshadow should not warn for shadowing an extern function

[Joseph S. Myers]

The code changes seem fine, but I don't think a testcase should depend on 
the details of what system strings.h declares (or even that it exists) 
like that.  If you want a system header declaration, add a header to the 
testsuite that uses #pragma GCC system_header to be sure it's handled as 
such, and include that header in the testcase; there are several existing 
examples of this in gcc.dg.

-- 
Joseph S. Myers
joseph@codesourcery.com