has ssp functions built-in" test via the TARGET_LIBC_PROVIDES_SSP environment
variable, to allow us to configure a cross-compiler appropriately without
having to try to find out by looking in the target's source directory.
Tweak our build to tell gcc that the ssp bits are now in libc.
The native compiler appears to already think that the ssp bits
live in libc, so no change appears to be needed there.
The autoconf-generated configure script will be committed separately shortly.
bd8f620dfb