linux-user vs system build options and libraries

[Michael Tokarev]

Hi!

I noticed that linux-user binaries, when built separately (with
--enable-linux-user --disable-system) or when built as part of
system build (--enable-linux-user --enable-system) differ in size
and link with significantly more libraries which they shouldn't
link with.  libnuma, liburing, libgnutls, - that's what gets linked,
just to name a few.  zlib is also there which most likely shouldn't
be.

Looking at the link lines I see a whole lot of various stuff being
linked, thankfully with --as-needed so most of it gets dropped by
the linker - these are stuff like pixman and many other things.

I already mentioned this before, - to me, whole meson.build options
thing is done a bit backwards: we use have_foo to mean NEED_foo, so
in quite a few places these meanings are wrongly intermixed.  Like,
if I try a linux-user build with --enable-spice, it will tried to
be used even if it makes no sense whatsoever.  We could check spice
and enable have_spice this way (or spice_libs=/spice_defs= etc),
but only use it in those build targets which actually need it.

Also, which might be a separate issue, --enable-plugins seems to
be in effect for linux-user too, which result in libgmodule being
linked with.

Also, there's no way to build/install just the linux-user parts
without, say, installing keymap files, docs are always built,
and so on..

It looks like we've quite barbarian build system still, even after
conversion to meson :)

/mjt