Fixed use of zlib

zlib might be provided in the LCG stack, so we have to properly find it instead of just getting it from the system.

• [2017-12-19 00:04] Validation started with lhcb-gaudi-head#1696
• [2017-12-19 00:04] Validation started with lhcb-gaudi-head-py3#8
• [2017-12-19 08:06] Validation started with lhcb-gaudi-merge#352
• [2017-12-19 09:31] Validation started with lhcb-gaudi-head#1697
• [2017-12-20 00:04] Validation started with lhcb-tdr-test#1
• [2017-12-20 00:04] Validation started with lhcb-gaudi-head-py3#9
• [2017-12-20 00:04] Validation started with lhcb-gaudi-head#1698

There is no log available for the test stage of the CI pipeline, it looks as if it just crashed before starting.

mentioned in merge request !561 (merged)

removed backport to v29 label

mentioned in commit 3c5d0e09

approved this merge request

merged

mentioned in commit f69b421d