Avoid shell parse errors after interrupt due to empty ``.

Avoid shell parse errors after interrupt due to empty ``.

* doc/autoconf.texi (Shell Substitutions): Document the issue.
* lib/m4sugar/m4sh.m4 (AS_VAR_IF): New function.
* lib/autoconf/functions.m4 (AC_CHECK_FUNC): Use it in place of
"test AS_VAR_GET([...]) = yes"
* lib/autoconf/general.m4 (AC_CHECK_FILE, AC_CHECK_DECL): Likewise.
* lib/autoconf/headers.m4 (_AC_CHECK_HEADER_MONGREL): Likewise.
(_AC_CHECK_HEADER_NEW, _AC_CHECK_HEADER_OLD): Likewise.
(_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.
* lib/autoconf/status.m4 (_AC_OUTPUT_FILES_PREPARE): Use
temporary variable to work around the issue.
* tests/foreign.at (Libtool): Quote result of command
substitution.

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