{lang}[*] flex: Make Bison a build dependency

[geimer]

As discussed on the mailing list a couple of weeks ago, Bison is not really necessary for using flex and can thus be specified as a build dependency. This PR implements this change for flex 2.6.0, and also adds a checksum for the source tarball. If this change is considered OK, I can prepare corresponding PRs for older versions of flex available in the repo.

Note: This change may potentially break packages that only (build-)depend on flex rather than depending on both flex and Bison. However, how many packages are affected is hard to predict.

edit: ref to thread on ML: https://lists.ugent.be/wws/arc/easybuild/2016-08/msg00017.html

[geimer]

Test report by @geimer
SUCCESS
Build succeeded for 6 out of 6 (6 easyconfigs in this PR)
swat - Linux debian 7.11, Intel(R) Xeon(R) CPU X5550 @ 2.67GHz, Python 2.7.3
See https://gist.github.com/d868c105039f2497f02d592cd37aeee3 for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 15 out of 15 (15 easyconfigs in this PR)
node2576.golett.os - Linux centos linux 7.2.1511, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz, Python 2.7.5
See https://gist.github.com/e6d51f029bb5341438cf1fae3ea4dd9b for a full test report.

[boegel]

We have 162 easyconfigs in our easyconfigs repo that list flex as a (build) dep, and each one of them also lists Bison as a (build) dep, so this should be OK.

Of course, we don't control all easyconfigs out there, but the upcoming EasyBuild v3.0 gives us an excuse to make this in theory backwards-incompatible change.

So: lgtm, thanks @geimer!

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 15 out of 15 (15 easyconfigs in this PR)
node2033.delcatty.os - Linux SL 6.8, Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz, Python 2.6.6
See https://gist.github.com/5ea49d78dc91ca248069c18b8aabe145 for a full test report.

[boegel]

Going in, thanks @geimer!