MINOR: build: allow packagers to specify the ssl cache size

This is done by passing the default value to SSLCACHESIZE in sessions.
User can use tune.sslcachesize to change this value.
By default, it is set to 20000 sessions as openssl internal cache size.
Currently, a session entry size is between 592 and 616 bytes depending on the arch.

include/common/defaults.h

@@ -191,4 +191,9 @@
 #define LISTEN_DEFAULT_CIPHERS NULL
 #endif
 
+/* ssl cache size */
+#ifndef SSLCACHESIZE
+#define SSLCACHESIZE 20000
+#endif
+
 #endif /* _COMMON_DEFAULTS_H */

src/haproxy.c

@@ -132,7 +132,7 @@ struct global global = {
 		.maxrewrite = MAXREWRITE,
 		.chksize = BUFSIZE,
 #ifdef USE_OPENSSL
-		.sslcachesize = 20000,
+		.sslcachesize = SSLCACHESIZE,
 #endif
 #ifdef USE_ZLIB
 		.zlibmemlevel = 8,