There is no support for thread_local on macOS, so variables with
thread local storage must use the ROOT macros for this purpose.

Qt's meta-object compiler (moc) only understands the -I option for
adding include paths.

This fixes the OS X warnings like this one:

 ld: warning: direct access in function 'XXX' from file
 'libLLVMScalarOpts.a(NewGVN.cpp.o)' to global weak
 symbol 'llvm::ReverseIterate<bool>::value' from file
 'interpreter/llvm/src/lib/libclingUtils.a(AST.cpp.o)'
 means the weak symbol cannot be overridden
 at runtime. This was likely caused by different translation
 units being compiled with different visibility settings.

I assume it's a compiler bug and it maybe fixes itself in LLVM 6.0
as this is quite recently introduced code, so let's go with the
most conservative fix and just disable this validation layer in LLVM
(that we don't use from what I can see).

A few minor tweaks in the language; note the existence of `clang-tidy`.

Mostly, this is a set of links to other, existing documentation.  However,
the GitHub web UI will recommend developers review this file before
submitting a pull-request.  It's a simple nudge to "do the right thing".

There was two problems with <iframe>:
  - In some cases an extra frame was drawn
  - Some browsers (chrome and Opera) , in some cases, did not display correctly the image (wrong size)

This was due to THashList::GetListForObject returning a list that contains more than just
the request objects.

This is useful know in order to avoid JSON conversion of colors (by JSROOT) if not needed.