Before this commit, the installopt target was a prerequisite of the
install target. This means it was run even when the native compilers
were not compiled.
Thus, the sequence
make -f Makefile.nt world
make -f Makefile.nt install
failed before this commit.
This commit fixes this by running the installopt target only if ocamlopt
exists, as is done for the Unix build system. It also removes the