Support -Wformat -Werror=format-security.

Check if gcc supports -Wformat -Werror=format-security and use it if so.
Fix m_gdbserver/remote-utils.c sr_perror call. Fixes Bug #334727.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13962 a5019735-40e9-0310-863c-91ae7b9d1cf9