build: make HAVE_O_CLOEXEC available outside LevelDB (bugfix)

configure.ac

@@ -1151,6 +1151,7 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <fcntl.h>]],
  [ AC_MSG_RESULT(yes); HAVE_O_CLOEXEC=1 ],
  [ AC_MSG_RESULT(no); HAVE_O_CLOEXEC=0 ]
 )
+AC_DEFINE_UNQUOTED([HAVE_O_CLOEXEC], [$HAVE_O_CLOEXEC], [Define to 1 if O_CLOEXEC flag is available.])
 
 dnl crc32c platform checks
 AC_MSG_CHECKING(for __builtin_prefetch)