Cycles: Tweak to the expf() speed workaround

Add compile-time check for particular glibc version which fixed the issue.
This makes it so own-compiled blender is the fastest in the world, and the
only issue remains what should we do for release builds.

After some discussion with Campbell we decided to keep it as is for now
because slowdown is not that much noticeable. We'll disable this workaround
for release builds when all the majority of the distros will switch to the
new version of glibc.

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