use a macro like CHECK_PRINTF for tor_sscanf
On OpenBSD/5.2 under gcc 4.2.1, the test case src/test/test_util.c warns on compile with
src/test/test_util.c: In function 'test_util_sscanf': src/test/test_util.c:1570: warning: Array size (20) smaller than format string size (1000000) (arg 3) src/test/test_util.c:1675: warning: Array size (20) smaller than format string size (1000) (arg 3)
This is an extra warning applied by OpenBSD for the platform's default compiler (see also ticket 7260 where this warning was also raised). I believe the cases used in the test are safe, so, to address this, let's start by removing the check for GNUC and instead use a macro like the existing CHECK_PRINTF macro, so we can condition it appropriately with a later patch.