Relax the compat CFLAGS

Checked with the Linaro TC from Ubuntu.

We only need to take care of the finite maths stuff, and one SSP related _chk thingy.
We're okay on the CXX front, and we don't need the ugly GNU_SOURCE workaround, since we're targeting something far newer than glibc 2.7.

In doubt, I applied those on Kobo devices too, but if they're running on >= glibc 2.15, that's not needed.