Newer GCC versions trigger warnings on older GLIBC versions. GLIBC 2.29
is warning free with GCC9. GLIBC 2.31 is warning free with GCC10. GLIBC
2.34 is warning free with GCC11.
Add milestones for 2.31 and 2.34 and use those to set the default value
for GLIBC_ENABLE_WERROR based on the GCC version.
Signed-off-by: Chris Packham <judge.packham@gmail.com>