Change the strerror_override handling to check $_JSON_C_STRERROR_OVERRIDE instead of using a variable, so we don't need to export it.

tests/test_basic.test

@@ -1,5 +1,7 @@
 #!/bin/sh
 
+export _JSON_C_STRERROR_ENABLE=1
+
 # Common definitions
 if test -z "$srcdir"; then
     srcdir="${0%/*}"