Allow inspecting AC_PREPROC_IFELSE output in true branch.

Allow inspecting AC_PREPROC_IFELSE output in true branch.

* lib/autoconf/general.m4 (_AC_PREPROC_IFELSE_BODY): Redirect
preprocessor output to conftest.i rather than /dev/null.
(_AC_PREPROC_IFELSE): Remove conftest.i in the postprocessing.
* tests/compile.at (Order of user actions and cleanup): Extend
test in the ACTION-IF-TRUE branch.
* doc/autoconf.texi (Running the Preprocessor): Document new
feature.
* NEWS: Update.

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