Update minimum required CMake version to 3.6

It's not a proper port to modern CMake, but allows a few simplifications here and there.

FYI @akraszna

• [2018-10-26 10:06] Validation started with lhcb-gaudi-merge#622
• [2018-10-26 14:48] Validation started with lhcb-lcg-dev4#696
• [2018-10-26 14:51] Validation started with lhcb-tdr-test#338
• [2018-10-27 00:07] Validation started with lhcb-lcg-dev4#697
• [2018-10-27 00:07] Validation started with lhcb-dd4hep#40
• [2018-10-27 00:09] Validation started with lhcb-lcg-dev3#691
• [2018-10-27 00:10] Validation started with lhcb-tdr-test#339
• [2018-10-27 00:13] Validation started with lhcb-sanitizers#46
• [2018-10-27 00:14] Validation started with lhcb-gaudi-head#2041
• [2018-10-27 09:48] Validation started with lhcb-lcg-dev3#692
• [2018-10-28 00:06] Validation started with lhcb-dd4hep#41
• [2018-10-28 00:06] Validation started with lhcb-gaudi-head#2042
• [2018-10-28 00:09] Validation started with lhcb-lcg-dev4#698
• [2018-10-28 00:11] Validation started with lhcb-lcg-dev3#693
• [2018-10-28 00:11] Validation started with lhcb-tdr-test#340
• [2018-10-28 00:12] Validation started with lhcb-sanitizers#47
• [2018-10-28 14:38] Validation started with lhcb-tdr-test#341
• [2018-10-29 00:06] Validation started with lhcb-dd4hep#42
• [2018-10-29 00:06] Validation started with lhcb-lcg-dev3#694
• [2018-10-29 00:06] Validation started with lhcb-lcg-dev4#699
• [2018-10-29 00:10] Validation started with lhcb-gaudi-head#2043
• [2018-10-29 00:11] Validation started with lhcb-sanitizers#48
• [2018-10-29 00:11] Validation started with lhcb-tdr-test#342
• [2018-10-30 00:06] Validation started with lhcb-lcg-dev4#700
• [2018-10-30 00:06] Validation started with lhcb-dd4hep#43
• [2018-10-30 00:08] Validation started with lhcb-lcg-dev3#695
• [2018-10-30 00:08] Validation started with lhcb-tdr-test#343
• [2018-10-30 00:10] Validation started with lhcb-gaudi-head#2044
• [2018-10-30 00:12] Validation started with lhcb-sanitizers#49
• [2018-10-31 00:06] Validation started with lhcb-lcg-dev4#701
• [2018-10-31 00:07] Validation started with lhcb-dd4hep#44
• [2018-10-31 00:08] Validation started with lhcb-lcg-dev3#696
• [2018-10-31 00:11] Validation started with lhcb-tdr-test#344
• [2018-10-31 00:14] Validation started with lhcb-gaudi-head#2045
• [2018-10-31 00:15] Validation started with lhcb-sanitizers#50
• [2018-10-31 11:42] Validation started with lhcb-lcg-dev3#697
• [2018-11-01 00:05] Validation started with lhcb-lcg-dev4#702
• [2018-11-01 00:05] Validation started with lhcb-dd4hep#45
• [2018-11-01 00:07] Validation started with lhcb-lcg-dev3#698
• [2018-11-01 00:10] Validation started with lhcb-tdr-test#345
• [2018-11-01 00:13] Validation started with lhcb-gaudi-head#2046
• [2018-11-01 00:17] Validation started with lhcb-sanitizers#51
• [2018-11-01 07:43] Validation started with lhcb-gaudi-head#2047

Weird that we're so much in sync Marco. I made a very similar update in our own code very recently.

atlas/atlasexternals!360 (merged)

So yes, version 3.6 seems to be a good minimum for us as well.

approved this merge request

merged

mentioned in commit 3b92cbe1