Include cstring for sanity_test_fdelt if required

SmartOS FD_ZERO is implemented in a way that requires
an external declaration of memcpy. We can not simply
include cstring in the existing file because
sanity_test_memcpy is attempting to replace memcpy, but we can do
so here, now that the fdelt test is split out.

configure.ac

@@ -758,6 +758,39 @@ fi
 
 AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h stdio.h stdlib.h unistd.h strings.h sys/types.h sys/stat.h sys/select.h sys/prctl.h])
 
+# FD_ZERO may be dependent on a declaration of memcpy, e.g. in SmartOS
+# check that it fails to build without memcpy, then that it builds with
+AC_MSG_CHECKING(FD_ZERO memcpy dependence)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+    #include <cstddef>
+    #if HAVE_SYS_SELECT_H
+    #include <sys/select.h>
+    #endif
+  ]],[[
+    #if HAVE_SYS_SELECT_H
+    fd_set fds;
+    FD_ZERO(&fds);
+    #endif
+  ]])],
+  [ AC_MSG_RESULT(no) ],
+  [
+      AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+          #include <cstring>
+          #if HAVE_SYS_SELECT_H
+          #include <sys/select.h>
+          #endif
+        ]], [[
+          #if HAVE_SYS_SELECT_H
+          fd_set fds;
+          FD_ZERO(&fds);
+          #endif
+        ]])],
+        [ AC_MSG_RESULT(yes); AC_DEFINE(HAVE_CSTRING_DEPENDENT_FD_ZERO, 1, [Define this symbol if FD_ZERO is dependent of a memcpy declaration being available]) ],
+        [ AC_MSG_ERROR(failed with cstring include) ]
+      )
+  ]
+)
+
 AC_CHECK_DECLS([getifaddrs, freeifaddrs],,,
     [#include <sys/types.h>
     #include <ifaddrs.h>]

src/compat/glibc_sanity_fdelt.cpp

@@ -7,6 +7,9 @@
 #endif
 
 #if defined(HAVE_SYS_SELECT_H)
+#ifdef HAVE_CSTRING_DEPENDENT_FD_ZERO
+#include <cstring>
+#endif
 #include <sys/select.h>
 
 // trigger: Call FD_SET to trigger __fdelt_chk. FORTIFY_SOURCE must be defined