AC_CHECK_PROG macro fails if REJECT value is set under Digital

AC_CHECK_PROG macro fails if REJECT value is set under Digital
Unix 4.0x and 5.0 because `"$@"' expands into an empty parameter
(instead of nothing) when there are no positional parameters.
From Nicolas Joly.

* acgeneral.m4 (AC_CHECK_PROG, AC_CHECK_PROGS): Prefer
`$$1' to `[$]$1', `$foo' to `[$]foo', `$[1]' to `[$]1', and
`${1+"$[@]"}' to `"[$]@"'.