Extend the previous patch allowing make -f Makefile.nt flexdll
install-flexdll not to require the install-flexdll stage.
OCAML_FLEXLINK is utilised as required to allow compilation of the entire
system using an in-tree compiled flexlink. The build process simply
required the flexdll target to appear before world.
opt.opt compiles a native code version of flexlink.exe as flexlink.opt.
install always installs flexlink.exe if it was compiled along with any
required .manifest files. It also installs the appropriate .o/.obj files
to $(INSTALL_LIBDIR).
At present, the bootstrapping is not extended to the Cygwin ports.