* Also check for `RapidJSON_INCLUDE_DIRS` which seems to be used now
* Do not add `-I` without subsequent path if the include directory
is missing/empty which would lead to the Clang error:
`error: unable to handle compilation, expected exactly one compiler job in ''`
Those libraries are direct dependencies and must be listed
explicitely when linking dynamically because transitive
dependencies of clangTooling are not pulled automatically
by CMake in this case.
This is required to use LLVM 6 which has switched to dynamic
libraries (at least under Arch Linux).
* The option `-resource-dir` must be specified to
point to Clang's resource directory containing built-in
header files. Then the massive warnings are gone.
* Setting the platform triple seems to be the right way
for cross-compilation. Note that the stdlib.h still needs
to be worked around.
* Use the c++utilities source directory for includes rather
than the locations where headers are goint to be installed
* Call find_package for c++utilities only once