configure.ac: link test for -fstack-protector-all

There are (broken) compilers out there that accept -fstack-protector-*
parameters, but do not provide the needed ssp library. As a result,
build does not fail, but link does. Use the local check_link_flag
autoconf function to correctly test for SSP availability.

Required for the Imagination Technologies Codescape IMG MIPS
toolchain, version 2015.10 (and who knows what else).

Fixes #744.

configure.ac

@@ -93,7 +93,7 @@ AS_IF([test x"$hardening" != x"no"], [
   # This one will likely succeed, even on platforms where it does nothing.
   check_cc_cxx_flag([-D_FORTIFY_SOURCE=2], [HARDEN_CFLAGS="$HARDEN_CFLAGS -D_FORTIFY_SOURCE=2"])
 
-  check_cc_cxx_flag([-fstack-protector-all],
+  check_link_flag([-fstack-protector-all],
    [HARDEN_CFLAGS="$HARDEN_CFLAGS -fstack-protector-all"
     check_cc_cxx_flag([-Wstack-protector], [HARDEN_CFLAGS="$HARDEN_CFLAGS -Wstack-protector"],
       [], [-fstack-protector-all])