build: lib/util/string_wrappers.h: fix optimisation check for clang

build: lib/util/string_wrappers.h: fix optimisation check for clang

Building with clang resulted in an error with undefined symbols
___unsafe_string_function_usage_here_size_t__ etc. Turns out the
existing check whether the compiler optimizes out functions doesn't
match the use case, ie the check said yes, but the functions were not
optimized out.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Oct 20 14:41:09 CEST 2014 on sn-devel-104