Forcing gcc to use /usr/gnu/bin/ld or /usr/gld rather than /usr/ld on Solaris 11.3 (Sparc)

I am trying to build libffi-3.4.5. The README file states that I must use GNU as and ld. On Solaris 11.3, these are either /usr/gas (or /usr/gnu/bin/as) and /usr/gld (or /usr/gnu/bin/ld). Short of recompiling gcc to use the GNU versions, how do I ask gcc to use them?

I tried AS=/usr/gnu/bin/as and LD=/usr/gnu/bin/ld. Both are ignored.