build: only generate soccr config.h if necessary

Without this, any rebuild fails with:

make[1]: Entering directory '/home/ubuntu/criu'
make[1]: Entering directory '/home/ubuntu/criu'
  GEN      .gitid
  GEN      criu/include/config.h
  GEN      /home/ubuntu/criu/criu/include/version.h
make[1]: 'compel/plugins/std.built-in.o' is up to date.
make[1]: Leaving directory '/home/ubuntu/criu'
ln: failed to create symbolic link '/home/ubuntu/criu/soccr/config.h': File exists
Makefile:217: recipe for target '/home/ubuntu/criu/soccr/config.h' failed
make: *** [/home/ubuntu/criu/soccr/config.h] Error 1
make: *** Waiting for unfinished jobs....
make[1]: Entering directory '/home/ubuntu/criu'
make[1]: 'compel/compel-host' is up to date.
make[1]: Leaving directory '/home/ubuntu/criu'
make[1]: Leaving directory '/home/ubuntu/criu'
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Makefile

@@ -207,7 +207,7 @@ $(eval $(call gen-built-in,images))
 SOCCR_A := soccr/libsoccr.a
 SOCCR_CONFIG := $(SRC_DIR)/soccr/config.h
 $(SOCCR_CONFIG): $(CONFIG_HEADER)
-	$(Q) ln -s ../$(CONFIG_HEADER_REL) $@
+	$(Q) test -f $@ || ln -s ../$(CONFIG_HEADER_REL) $@
 soccr/%: $(SOCCR_CONFIG) .FORCE
 	$(Q) $(MAKE) $(build)=soccr $@
 soccr/built-in.o: $(SOCCR_CONFIG) .FORCE