BUILD: report zlib support in haproxy -vv

Compression algorithms are not always supported depending on build options.
"haproxy -vv" now reports if zlib is supported and lists compression algorithms
also supported.
diff --git a/src/compression.c b/src/compression.c
index 983e771..300a0bd 100644
--- a/src/compression.c
+++ b/src/compression.c
@@ -50,7 +50,7 @@
 #endif
 
 
-static const struct comp_algo comp_algos[] =
+const struct comp_algo comp_algos[] =
 {
 	{ "identity", 8, identity_init, identity_add_data, identity_flush, identity_reset, identity_end },
 #ifdef USE_ZLIB