Clean up memset_s() detection a bit more.

Clean up memset_s() detection a bit more.

Definining __STDC_WANT_LIB_EXT1__ to get memset_s() is required by the C11
standard, not a Solaris-ism. It's no use just to check for its presence
in the library with AC_CHECK_FUNC() if it isn't going to compile, so make
sure we check for it with AC_LINK_IFELSE() *and* with the warning flags
that might include -Werror-implicit-function-declarations.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>