> so there is a Xt and a .WAIT in SUBDIR before Xmuu - but make does not go
> into that directory (or so it seems).

Well - it would do that later!

After a complete (no -j) build, the make -j 1 there does:

all ===> X11
all ===> X11/dynamic
all ===> X11/static
all ===> ICE
all ===> Xau
all ===> SM
all ===> Xext
all ===> Xft
all ===> Xft1
all ===> Xfontcache
all ===> Xi
all ===> Xinerama
all ===> Xmuu
all ===> Xp
all ===> Xrandr
all ===> Xss
all ===> Xtst
all ===> Xv
all ===> XvMC
all ===> Xxf86dga
all ===> Xxf86misc
all ===> Xxf86vm
all ===> dmx
all ===> dpstk
all ===> expat
all ===> font
all ===> font/stubs
all ===> fontconfig
all ===> fontconfig/fontconfig
all ===> fontconfig/src
all ===> fontconfig/etc
all ===> fontenc
all ===> freetype2
all ===> freetype2/freetype
all ===> freetype2/freetype/cache
all ===> freetype2/freetype/config
all ===> freetype2/freetype/internal
all ===> freetype2/freetype/internal/services
all ===> lbxutil
all ===> psres
all ===> oldX
all ===> xkbfile
all ===> xkbui
all ===> xtrans
all ===> xlibi18n
all ===> xlibi18n/im
all ===> xlibi18n/lc
all ===> xlibi18n/lc/Utf8
all ===> xlibi18n/lc/def
all ===> xlibi18n/lc/gen
all ===> xlibi18n/lc/xlocale
all ===> xlibi18n/om
all ===> Xt
all ===> Xpm
all ===> Xrender
all ===> GL
all ===> Xmu
all ===> dps
all ===> FS
all ===> GLU
all ===> GLw
all ===> OSmesa
all ===> XRes
all ===> XTrap
all ===> Xaw
all ===> Xaw6
all ===> Xcursor
all ===> Xdmcp

Note that all subdirs between the second and the last .WAIT have been delayed.
I have no idea, why.

Martin