Conditionally enable SSP

GCC has SSP built-in and will still get it enabled as before.
uClibc provides SSP via libssp, but only if it was compiled
with SSP support, i.e. if SSP_SUPPORT=y is passed to ./do.

SSP support is not ubiquituous on OpenWRT, and with this
change we're able to prepare OPKG packages for both cjdns-ssp
and cjdns-nossp, and a metapackage selecting either one based
on whether SSP is available.