Allow AS_IF in shell lists.

Allow AS_IF in shell lists.

* lib/m4sugar/m4sh.m4 (AS_IF): Always execute test, in case of
side effects.  Allow continuation of script on same line as fi.
(_AS_DETECT_BETTER_SHELL): Adjust clients.
(AS_VAR_IF, AS_VAR_SET_IF): For now, supply newline no longer
given by AS_IF.
* lib/autoconf/c.m4 (_AC_PROG_PREPROC_WORKS_IFELSE): Likewise.
* lib/autoconf/general.m4 (_AC_ENABLE_IF): Likewise.
(AC_EGREP_CPP, _AC_RUN_IFELSE): Adjust client.
* lib/autoconf/libs.m4 (AC_SEARCH_LIBS): Likewise.
* doc/autoconf.texi (Common Shell Constructs) <AS_IF>: Fix typo.
(Polymorphic Variables): Move mention of dnl to the only two
AS_VAR functions that need it.

Signed-off-by: Eric Blake <ebb9@byu.net>