rules.mak: fix $(obj) to a real relative path

Makefile.target includes rule.mak and unnested common-obj-y, then prefix
them with '../', this will ignore object specific QEMU_CFLAGS in subdir
Makefile.objs:

    $(obj)/curl.o: QEMU_CFLAGS += $(CURL_CFLAGS)

Because $(obj) here is './block', instead of '../block'. This doesn't
hurt compiling because we basically build all .o from top Makefile,
before entering Makefile.target, but it will affact arriving per-object
libs support.

The starting point of $(obj) is passed in as argument of unnest-vars, as
well as nested variables, so that different Makefiles can pass in a
right value.

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

configure

@@ -2360,6 +2360,7 @@ fi
 if $pkg_config --atleast-version=$glib_req_ver gthread-2.0; then
     glib_cflags=`$pkg_config --cflags gthread-2.0`
     glib_libs=`$pkg_config --libs gthread-2.0`
+    CFLAGS="$glib_cflags $CFLAGS"
     LIBS="$glib_libs $LIBS"
     libs_qga="$glib_libs $libs_qga"
 else