build: Always use $(LN_S) and $(MKDIR_P)

autotools provide those for our convenience, so let's use them
everywhere instead of mixing in native command invocation.

Makefile.am

@@ -68,7 +68,7 @@ rpm: clean
 check-local: all tests
 
 cov: clean-cov
-	mkdir $(top_builddir)/coverage
+	$(MKDIR_P) $(top_builddir)/coverage
 	$(LCOV) -c -o $(top_builddir)/coverage/libvirt.info.tmp \
 	  -d $(top_builddir)/src  -d $(top_builddir)/daemon \
 	  -d $(top_builddir)/tests

src/Makefile.am

@@ -3079,7 +3079,7 @@ if WITH_NETWORK
 	      $(DESTDIR)$(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) ../default.xml \
 	    $(DESTDIR)$(confdir)/qemu/networks/autostart/default.xml
 endif WITH_NETWORK