Fix configure script build with autoconf < 2.63

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)

changes/bug12693

+  o Build fixes:
+    - Allow our configure script to build correctly with autoconf 2.62
+      again. Fixes bug 12693; bugfix on 0.2.5.2-alpha.

configure.ac

@@ -626,11 +626,12 @@ if test x$enable_gcc_hardening != xno; then
     TOR_CHECK_CFLAGS(-fstack-protector-all, also_link)
     AS_VAR_PUSHDEF([can_compile], [tor_cv_cflags_-fstack-protector-all])
     AS_VAR_PUSHDEF([can_link], [tor_can_link_-fstack-protector-all])
+m4_ifdef([AS_VAR_IF],[
     AS_VAR_IF(can_compile, [yes],
         AS_VAR_IF(can_link, [yes],
                   [],
                   AC_MSG_ERROR([We tried to build with stack protection; it looks like your compiler supports it but your libc does not provide it. Are you missing libssp? (You can --disable-gcc-hardening to ignore this error.)]))
-        )
+        )])
     AS_VAR_POPDEF([can_link])
     AS_VAR_POPDEF([can_compile])
     TOR_CHECK_CFLAGS(-Wstack-protector)