bodge around https://gitlab.isc.org/isc-projects/kea/-/issues/2970

Index: src/lib/eval/lexer.cc
--- src/lib/eval/lexer.cc.orig
+++ src/lib/eval/lexer.cc
@@ -312,7 +312,7 @@
 
 /* C99 systems have <inttypes.h>. Non-C99 systems may or may not. */
 
-#if defined (__STDC_VERSION__) && __STDC_VERSION__ >= 199901L
+#if defined(__OpenBSD__) || (defined (__STDC_VERSION__) && __STDC_VERSION__ >= 199901L)
 
 /* C99 says to define __STDC_LIMIT_MACROS before including stdint.h,
  * if you want the limit (max/min) macros for int types.