Changes in the CMake part to make it work for the LCG stack

These are a number of small changes in the CMake part that are needed to add Garfield++ into the LCG software stack

• The header guards HEP_SYSTEM_OF_UNITS_H/HEP_PHYSICAL_CONSTANTS_H clashed with the CLHEP ones
• Installation of examples under the /examples directory instead of /bin
• Reduced the list of ROOT libraries to be linked to the minimal ones
• Make use of the variable CMAKE_CXX_STANDARD to set the C++ standard instead of compilation flags
• Install header files under include/Garfield

added 1 commit

• e148b480 - CMAKE_CXX_STANDARD can be defined externally

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assigned to @hschindl

merged

mentioned in commit 76ec4fbd