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.