update CMake build dependency for Eigen 3.3.4 + 3.3.5 using system toolchain

[boegel]

(created using eb --new-pr)
GCC 6.4 doesn't work on CentOS 8.x, but GCC 7.3 does...

[boegel]

@boegelbot please test @ generoso

[boegelbot]

@boegel: Request for testing this PR well received on generoso

PR test command 'EB_PR=13889 EB_ARGS= /apps/slurm/default/bin/sbatch --job-name test_PR_13889 --ntasks=4 ~/boegelbot/eb_from_pr_upload_generoso.sh' executed!

• exit code: 0
• output:

Submitted batch job 18266

Test results coming soon (I hope)...

- notification for comment with ID 912318623 processed

Message to humans: this is just bookkeeping information for me,
it is of no use to you (unless you think I have a bug, which I don't).

[boegelbot]

Test report by @boegelbot
SUCCESS
Build succeeded for 2 out of 2 (2 easyconfigs in total)
generoso-c1-s-6 - Linux centos linux 8.2.2004, x86_64, Intel(R) Xeon(R) CPU E5-2667 v3 @ 3.20GHz (haswell), Python 3.6.8
See https://gist.github.com/166ceefb4f97d708fbc2f8fd518895b4 for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 2 out of 2 (2 easyconfigs in total)
node2604.swalot.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz (haswell), Python 3.6.8
See https://gist.github.com/a961a7c14dad999dae4a120d96046033 for a full test report.

[Micket]

lgtm

[Micket]

Test report by @Micket
SUCCESS
Build succeeded for 2 out of 2 (2 easyconfigs in total)
alvis-c1 - Linux centos linux 7.9.2009, x86_64, Intel Xeon Processor (Skylake), Python 3.6.8
See https://gist.github.com/b31cab791b146476a8334769f15ebd8b for a full test report.

[Micket]

Going in, thanks @boegel!