Cherry-pick "Merge branch 'PPC64-fixes-master-20181207' into 'master'" into 1.0

PowerPC Fixes, master branch (2018.12.12.)

See merge request !401 (merged)

(cherry picked from commit c72178d0)

• 60da288a Made ROOT build successfully on PowerPC64.
• a51ef7d0 Updated AnalysisBaseExternals to only attempt the build of dcap on x86_64.
• c1d91f42 Disabled the build of BAT and KLFitter in C++17 mode.