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.