http://bugs.gentoo.org/422789

--- src/init_consts.cc
+++ src/init_consts.cc
@@ -27,6 +27,10 @@
 #include "debug.h"
 #endif
 
+#if USE_MMAP
+#include <unistd.h> /* sysconf */
+#endif
+
 #include "globals.h"
 #include "superblock.h"
 #include "forward_declarations.h"