In fact, rather than waste CPU cycles recompiling, I copied the isl and mpfr4 libraries directly from there.
Finally, thanks also to ppa:ubuntu-toolchain-r/test for the GCC-5.3.0 backport, with which I could build GCC-5.3.1 from Xenial.
/usr/bin/ps2pdf checking Whether to build documentation... What exactly do you need to do to install the 2.9.1 driver from source (or otherwise)?
I have a feeling that I'm missing a step somewhere.
/usr/bin/install -c checking whether build environment is sane...
i686-pc-linux-gnu checking for style of include used by make... gcc checking for C compiler default output file name...