Add FindCURL.cmake, 1.0 branch (2019.03.13.)
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.
Conflicts:
Build/AtlasLCG/LCGConfig.cmake
Note that this is a manual cherry-pick of !423 (merged).