mh bindir doesn't propagate to subdirs

There seems to be a glitch in the build system so that the value of 
bindir set in mh/Makefile.am doesn't propagate to the subdirs etc and tests.

I built using --with-mh-bindir=/usr/libexec/mu-mh.  The resulting 
mh/Makefile correctly had

   bindir = /usr/libexec/mu-mh

but the Makefiles in etc and test had

   bindir = ${exec_prefix}/bin

As a consequence, the mailutils-mh.el file that got built contained

   (setq mh-progs "/usr/bin")

instead of

   (setq mh-progs "/usr/libexec/mu-mh")

I'm sure there's a simple fix for this, but my knowledge of Automake is 
not sufficient for me to provide a patch.

Ken