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.

Backport of !553 (merged)

changed the description

changed target branch from master to v29-patches

• [2017-12-19 10:58] Validation started with lhcb-gaudi-merge#360

added 26 commits

• b8b84997...2fd25b6f - 17 commits from branch gaudi:v29-patches
• fe55e97e - GaudiAlg FunctionalDetails apply range size check in debug builds only (mr !529 (merged))
• 03f32eb5 - GaudiAlg FunctionalDetails apply range size check in debug builds only (mr !556 (merged))
• 2bda8835 - Remove IssueSeverity support from StatusCode (mr !477 (merged))
• 0fe7ba93 - Remove IssueSeverity support from StatusCode (mr !557 (merged))
• 5932bf2f - Fix unnamed lock in example code (mr !487 (merged))
• 6cf7c619 - Fix unnamed lock in example code (mr !559 (merged))
• 708def06 - StatusCode check fixes, master branch (2017.10.26.)
• 721fe22b - StatusCode check fixes (mr !562 (merged))
• 5dff4946 - Fixed use of zlib

Compare with previous version

mentioned in commit 3c5d0e09

merged