Looks like all your problems stem from your gcc distribution. The CMake file shipped with ROOT copies the wchar.h file from your distribution before compilation (core/clingutils/CMakeLists.txt):
# These headers do not have complete include guards on all platforms we
# support. This means that the PCH cannot provide their representation at
# runtime and clang will hit disk, triggering a possible incompatibility
# of that file in build-time versus run-time (different length etc).
# Capture their build-time version here, and inject it into runtime.
foreach(file wchar.h bits/stat.h bits/time.h)
if(EXISTS /usr/include/${file})
list(APPEND copy_commands COMMAND ${CMAKE_COMMAND} -E copy /usr/include/${file} ${CMAKE_BINARY_DIR}/etc/cling/lib/clang/${LLVM_VERSION}/include/${file})
endif()
endforeach()
Maybe you have clashing gcc packages? (EDIT: This was the issue with OP.) Have you tried building something simple that use the string header?