Testsuite coverage for AC_CACHE_VAL and caching semantics.

Testsuite coverage for AC_CACHE_VAL and caching semantics.

* tests/base.at (AC_CACHE_CHECK): Extend test.
(AC_CACHE_LOAD): New test.
* tests/torture.at (Configuring subdirectories): Also test
--config-cache with AC_CONFIG_SUBDIRS.
* doc/autoconf.texi (Caching Results): Annotate code snippets
which are tested in the test suite.
(Cache Files): Documented cache variables may be used on the
configure command line to override individual entries in the
cache file.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>