docs: document AM_MISSING_PROG Addresses automake bug#11793. This macro has been present for a long time, and coreutils has been relying on it despite no documentation, which argues that it is stable enough to be worth documenting. Furthermore, since we are hoping to change our preferred invocation from 'missing --run program' to 'missing program' in a future version of automake, we need a way for packages to consistently get the preferred invocation form rather than open-coding a call to 'missing'. In particular, the Autoconf manual would love to recommend this macro when discussing how to integrate an autotest suite with automake. * doc/automake.texi (Public Macros): Document AM_MISSING_PROG. Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>