To build Autotoolized fd.o X11 stuff from scratch I need to kludge it like this
for the libXt (v.0.1.4):
--
pre-configure:
        cp -vu files/Xalloca.h $(prefix)/include/X11/
        # generate configure with header fix
        ln -sf libXt-0.1.4 X11
        cd libXt-0.1.4; \
        export CPPFLAGS="-I../ -Iinclude"; \
        sh ./autogen.sh --prefix=$(prefix)

post-install:
        # install missing header files
        cp -vu libXt-0.1.4/{ObjectP,Shell}.h $(prefix)/include/X11/
--

Xalloca.h should probably be in the xproto package, libXt should have
it's includes where it finds them, and it should install all the headers
required by successive X packages (Xmu/Xaw in this case)