m4sh: require that 'test -x' works

m4sh: require that 'test -x' works

4.3BSD is no longer a reasonable portability target; and we are
pretty sure that these days we can find at least one shell on any
platform that supports 'test -x'.  Drop a horribly unsafe use of
eval as a result. :)

Libtool still uses $as_executable_p without so much as calling
either AS_TEST_X or AS_EXECUTABLE_P; even though the latter has
existed, although undocumented, since at least 2.59; furthermore,
libtool uses it in a context where filtering out directories
would have been desirable.  Shame on them.

* lib/m4sugar/m4sh.m4 (_AS_TEST_X_WORKS): New probe.
(AS_SHELL_SANITIZE, AS_INIT): Use it in shell searching.
(AS_TEST_X, AS_EXECUTABLE_P): Simplify.

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