rules.mak: Force CFLAGS for all objects in DSO

Because of the trick of process-archive-undefs, all .mo objects, even
with --enable-modules, are dependencies of executables.

This breaks CFLAGS propogation because the compiling of module object
will happen too early before building for DSO.

With GCC 5, the linking would fail because .o doesn't have -fPIC. Also,
BUILD_DSO will be missed. (module-common.o will have it, so the stamp
symbol was still liked in .so).

Fix the problem by forcing the CFLAGS on individual .o-cflags during
unnest-vars.

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Fam Zheng <famz@redhat.com>
Cc: qemu-stable@nongnu.org # 2.3
Message-Id: <1430981715-31465-1-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>