Handle $srcdir/lib/my-syscalls.h using lappend_include_dir.
Tested on x86_64-linux.
 #include <string.h>
 #include <pthread.h>
 
-#include "../lib/my-syscalls.h"
+#include "my-syscalls.h"
 
 #if (!defined(LEADER_DOES_EXEC) && !defined(OTHER_DOES_EXEC) \
      || defined(LEADER_DOES_EXEC) && defined(OTHER_DOES_EXEC))
 
     set execr_opts [list debug]
     set execd_opts [list debug]
 
+    lappend_include_file execr_opts $::srcdir/lib/my-syscalls.h
+
     if { $different_text_segments } {
        lappend execr_opts "text_segment=0x600000"
        lappend execd_opts "text_segment=0x800000"