Allow AS_VAR_IF in shell lists.

Allow AS_VAR_IF in shell lists.

* lib/m4sugar/m4sh.m4 (AS_VAR_IF): Allow continuation of line.
* lib/autoconf/functions.m4 (AC_CHECK_FUNC): Adjust clients.
* lib/autoconf/general.m4 (AC_CHECK_FILE, AC_CHECK_DECL):
Likewise.
* lib/autoconf/headers.m4 (_AC_CHECK_HEADER_MONGREL)
(_AC_CHECK_HEADER_NEW, _AC_CHECK_HEADER_OLD)
(_AC_CHECK_HEADER_DIRENT): Likewise.
* lib/autoconf/libs.m4 (AC_CHECK_LIB): Likewise.
* lib/autoconf/types.m4 (_AC_CHECK_TYPE_NEW, AC_CHECK_MEMBER):
Likewise.
* tests/m4sh.at (AS@&t@_VAR basics): Enhance test.

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