The vendored copy of Google's cpp-btree relies on <cstdint> being pulled in
transitively.  libstdc++ 15 no longer does so, which makes uint8_t/uint16_t
undeclared, and the resulting fallback to int trips the kNodeValues
COMPILE_ASSERT with a "1 << 32" shift.

diff --git a/include/btree.h b/include/btree.h
--- a/include/btree.h
+++ b/include/btree.h
@@ -104,6 +104,7 @@
 #include <stddef.h>
 #include <string.h>
 #include <sys/types.h>
+#include <cstdint>
 #include <algorithm>
 #include <functional>
 #include <iostream>