title: cached files/functions when running ./configure
body: <p>Any time I run configure I see output like </p>
  
  <pre><code> 0:20.96 checking for res_ninit()... (cached) yes
   0:20.96 checking for nl_langinfo and CODESET... (cached) yes
   0:20.96 checking for an implementation of va_copy()... (cached) yes
   0:20.96 checking whether va_list can be copied by value... (cached) no
   0:20.97 checking whether the C++ "using" keyword resolves ambiguity... (cached) yes
   0:20.97 checking for C++ dynamic_cast to void*... (cached) yes
   0:20.97 checking whether C++ requires implementation of unused virtual methods... (cached) yes
   0:20.97 checking for trouble comparing to zero near std::operator!=()... (cached) no
   0:20.97 checking for __thread keyword for TLS variables... (cached) yes
   0:20.97 checking for __attribute__((always_inline))... (cached) yes
   0:20.97 checking for __attribute__((malloc))... (cached) yes
   0:20.97 checking for __attribute__((warn_unused_result))... (cached) yes
   0:20.97 checking for LC_MESSAGES... (cached) yes
   0:20.97 checking for localeconv... (cached) yes
  </code></pre>
  
  <p>I would want to know what the cached output means.</p>