LDAP: Don't use macro _XOPEN_SOURCE for extra features
    
    We defined macro _XOPEN_SOURCE before time.h, because we need function strptime
    The problem is with undef after including header time.h
    The macro _XOPEN_SOURCE can be defined on some platforms
    and undef can cause problems.
    
    We detect all necessary feature macros in configure script
    using AC_USE_SYSTEM_EXTENSIONS or AC_GNU_SOURCE.
    It is better to include header file config.h
    instead of defining macro _XOPEN_SOURCE
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 80c092f94a7ac24ea00e560201e37ae27cfbf665)