We added some AS_VAR_IF-based checks to detect whether we have
managed to compile (but not link) with stack-protector.  On autoconf
before 2.63, we don't have AS_VAR_IF, so we just have to let the
user get a compile error rather than a helpful "find libssp" error.

Fixes bug 12693; bugfix on 0.2.5.2-alpha (commit 21ac2928)