BUILD: makefile: do not erase build options for some build options

BUILD: makefile: do not erase build options for some build options

One painfully annoying thing with the build options change detection
is that they get rebuild for about everything except when the build
target is exactly "reg-tests". But in practice every time reg tests
are run we end up having to experience a full rebuild because the
reg-tests script runs "make version" which is sufficient to refresh
the file.

There are two issues here. The first one is that we ought to skip all
targets that do not make use of the build options. This includes all
the tools such as "flags" for example, or utility targets like "tags",
"help" or "version". The second issue is that with most of these extra
targets we do not set the TARGET variable, and that one is used when
creating the build_opts file, so let's preserve the file when TARGET
is not set.

Now it's possible to re-run a make after a make reg-tests without having
to rebuild the whole project.