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/CMakeLists.txt

@@ -37,6 +37,10 @@ foreach(TESTNAME
 	test_object_iterator)
 
 add_executable(${TESTNAME} ${TESTNAME}.c)
+if(${TESTNAME} STREQUAL test_util_file)
+# For output consistency, we need _json_c_strerror() in some tests:
+target_sources(${TESTNAME} PRIVATE ../strerror_override.c)
+endif()
 add_test(NAME ${TESTNAME} COMMAND ${PROJECT_SOURCE_DIR}/tests/${TESTNAME}.test)
 
 # XXX using the non-target_ versions of these doesn't work :(