Configure: improved dependencies tracking.

Using nginx Makefile as a dependency for libnjs.a to ensure
libnjs.a is rebuilt after auto/configure invocation.
diff --git a/nginx/config.make b/nginx/config.make
index 0ad176d..09ce410 100644
--- a/nginx/config.make
+++ b/nginx/config.make
@@ -1,6 +1,6 @@
 cat << END                                            >> $NGX_MAKEFILE
 
-$ngx_addon_dir/../build/libnjs.a:
+$ngx_addon_dir/../build/libnjs.a: $NGX_MAKEFILE
 	cd $ngx_addon_dir/.. \\
 	&& if [ -f build/Makefile ]; then \$(MAKE) clean; fi \\
 	&& CFLAGS="\$(CFLAGS)" CC="\$(CC)" ./configure \\