All Meson executables should specify their dependencies explicitly, either
directly or indirectly via declare_dependency.  Makefiles instead did
not propagate dependencies correctly from static libraries, for example.
Therefore, flags for dependencies need not be included in QEMU_CFLAGS.
LIBS is not used at all, so drop that one as well.

In a few cases the dependencies were not yet specified, so add them.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>