build: Overwrite existing symbolic links

The current rule fails if the target already exists:

  cd /home/jenkins/build/libvirt/lib && \
    ln -s libnss_libvirt.so.1 nss_libvirt.so.1
  ln: nss_libvirt.so.1: File exists
  Makefile:3357: recipe for target 'install-exec-hook' failed

However, all other rules concerned with installation are
idempotent and will happily overwrite an existing target,
so this one should as well.

src/Makefile.am

@@ -3078,8 +3078,7 @@ if WITH_NETWORK
 	    cp $(DESTDIR)$(confdir)/qemu/networks/default.xml.t \
 	      $(DESTDIR)$(confdir)/qemu/networks/default.xml && \
 	    rm $(DESTDIR)$(confdir)/qemu/networks/default.xml.t; }
-	test -e $(DESTDIR)$(confdir)/qemu/networks/autostart/default.xml || \
-	   $(LN_S) ../default.xml \
+	$(LN_S) -f ../default.xml \
 	    $(DESTDIR)$(confdir)/qemu/networks/autostart/default.xml
 endif WITH_NETWORK
 

tools/Makefile.am

@@ -424,7 +424,7 @@ NSS_SO_VER = 1
 
 install-exec-hook:
 	cd $(DESTDIR)$(libdir) && \
-	  $(LN_S) libnss_libvirt.so.$(NSS_SO_VER) nss_libvirt.so.$(NSS_SO_VER)
+	  $(LN_S) -f libnss_libvirt.so.$(NSS_SO_VER) nss_libvirt.so.$(NSS_SO_VER)
 
 uninstall-local:
 	-rm -f $(DESTDIR)$(libdir)/nss_libvirt.so.$(NSS_SO_VER)