Forwared to upstream and fixed in pdal>2.2.0
---
diff --git a/filters/private/expr/Lexer.cpp b/filters/private/expr/Lexer.cpp
index 0c3dbe6b3..854d4c693 100644
--- a/filters/private/expr/Lexer.cpp
+++ b/filters/private/expr/Lexer.cpp
@@ -1,6 +1,7 @@
 
 #include <cassert>
 #include <cctype>
+#include <cstdlib>
 
 #include "Lexer.hpp"