gcc of OpenBSD does not like the original assignment.
Newer versions of gcc seem to have no problem.
The explicit cast fixes this error for gcc 3.3.5 in OpenBSD

--- src/mem.cc.orig	Tue Jun  1 17:24:03 2010
+++ src/mem.cc	Tue Jun  1 17:24:22 2010
@@ -356,7 +356,7 @@
     else {
 	if (Config.MemPools.limit == 0)
 	    debugs(13, 1, "memory_pools_limit 0 has been chagned to memory_pools_limit none. Please update your config");
-        new_pool_limit = -1;
+        new_pool_limit = static_cast<size_t>(-1);
     }
 
 #if 0