Search notes:

AC_CHECK_PROGS

AC_CHECK_PROGS(var, progs, val-if-not-found)

Sets $var to the first program found in the progs list or sets it to val-if-not-found otherwise.

AC_CHECK_PROGS([TAR], [tar gtar], [na])
if test "$TAR" = na; then
   AC_MSG_ERROR([tar or gtar needed]) 
fi

See also