xrootd tweaks
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This merge request is automatically copied from !659 (merged) orginally made by @ellert
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The new xrootd DMC uses new features in the xrootd cliet library and
therefore requires xrootd >= 4.5.0 to build.
Xrootd 4.9.0 and later requires additional compiler flags when used with older compilers (i.e. in EPEL).