m4sh: make AS_EXECUTABLE_P public

m4sh: make AS_EXECUTABLE_P public

In the process of making it public, factor it into a reusable
function.  This makes constructs like AC_CHECK_PROGRAM smaller,
as well as making libtool's naughty use of $as_executable_p safer.

* lib/m4sugar/m4sh.m4 (_AS_TEST_PREPARE): Add a shell function.
(AS_EXECUTABLE_P): Forward to shell function.
* doc/autoconf.texi (Common Shell Constructs): Document it.
* NEWS: Mention this.

Signed-off-by: Eric Blake <eblake@redhat.com>