( Caching Results

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 6.3 Caching Results
 To avoid checking for the same features repeatedly in various
 `configure' scripts (or repeated runs of one script), `configure' saves
 the results of many of its checks in a "cache file".  If, when a
 `configure' script runs, it finds a cache file, it reads from it the
 results from previous runs and avoids rerunning those checks.  As a
 result, `configure' can run much faster than if it had to perform all
 of the checks every time.
      Ensure that the results of the check identified by CACHE-ID are
      available.  If the results of the check were in the cache file
      that was read, and `configure' was not given the `--quiet' or
      `--silent' option, print a message saying that the result was
      cached; otherwise, run the shell commands COMMANDS-TO-SET-IT.
      Those commands should have no side effects except for setting the
      variable CACHE-ID.  In particular, they should not call
      `AC_DEFINE'; the code that follows the call to `AC_CACHE_VAL'
      should do that, based on the cached value.  Also, they should not
      print any messages, for example with `AC_MSG_CHECKING'; do that
      before calling `AC_CACHE_VAL', so the messages are printed
      regardless of whether the results of the check are retrieved from
      the cache or determined by running the shell commands.  If the
      shell commands are run to determine the value, the value will be
      saved in the cache file just before `configure' creates its output
      files.   Cache Variable Names, for how to choose the name
      of the CACHE-ID variable.
      A wrapper for `AC_CACHE_VAL' that takes care of printing the
      messages.  This macro provides a convenient shorthand for the most
      common way to use these macros.  It calls `AC_MSG_CHECKING' for
      MESSAGE, then `AC_CACHE_VAL' with the CACHE-ID and COMMANDS
      arguments, and `AC_MSG_RESULT' with CACHE-ID.
  -- Macro: AC_CACHE_LOAD
      Loads values from existing cache file, or creates a new cache file
      if a cache file is not found.  Called automatically from `AC_INIT'.
  -- Macro: AC_CACHE_SAVE
      Flushes all cached values to the cache file.  Called automatically
      from `AC_OUTPUT', but it can be quite useful to call
      `AC_CACHE_SAVE' at key points in  Doing so
      checkpoints the cache in case of an early configure script abort.


* Cache Variable Names        Shell variables used in caches.
* Cache Files                 Files `configure' uses for caching.
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