Implement CTestCustom.cmake files in sources. Attempt to fix ROOT-6529.
Applied patch from Ben Morgan (ROOT-6512 - RootConfiguration.cmake hardcodes paths into installed ROOTConfig.cmake)
Implement CTestCustom.cmake files in sources. Attempt to fix ROOT-6529
Correct handling of rootrc Unix.*.Root.UseThreads

InitThreads requires the interpreter to be initalized.

Also extend it to actually initialize the TThread mechanism (it was previously
just loading the library and creating the TClass object for TThread).

This fixes ROOT-6379
Remove last static from zip/inc/Bits.h
Add missing error flag initialization and setting
Remove unused #define
introduce RZip.h with ROOT-specific functions, do not expose other includes from core/zip/inc
introduce RZip.h with ROOT-specific functions, do not expose other includes from core/zip/inc
add RZip.cxx into cmake file
Move 'private' headers to src directory
Fix CMake broken by previous commit