MINOR: build: allow packagers to specify the default maxzlibmem

This is done by passing the default value to DEFAULT_MAXZLIBMEM in megs.
diff --git a/src/haproxy.c b/src/haproxy.c
index 32db281..5e6ee40 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -114,7 +114,11 @@
 struct global global = {
 	.req_count = 0,
 	.logsrvs = LIST_HEAD_INIT(global.logsrvs),
+#ifdef DEFAULT_MAXZLIBMEM
+	.maxzlibmem = DEFAULT_MAXZLIBMEM,
+#else
 	.maxzlibmem = 0,
+#endif
 	.comp_rate_lim = 0,
 	.unix_bind = {
 		 .ux = {