Add FindCURL.cmake, master branch (2019.01.17.)

Introduced FindCURL.cmake to find/use curl correctly on CentOS 7.

Note that only the CentOS 7 LCG releases include curl. On SLC6 we always pick it up from the OS, that's why the find module was not part of AtlasLCG so far.

This is to address ATLINFR-2820.

Note to self: Will need to do some testing by hand before merging this in...

AE Build SUCCESS
Build logfiles are available at Jenkins [AE-MERGE-REQUEST #684]

resolved all discussions

merged

mentioned in commit 1c0d0b06

mentioned in merge request !463 (merged)