When building modules with clang from the trunk we need to build with gcc's
libstdc++. Apparently this can be done in clang in 2 ways:
  1. Building clang with -DGCC_INSTALL_PREFIX=$(dirname `which gcc`)/../
     This would hardcode the patch to the gcc toolchain when compiling clang.
  2. Pass on every compiler invocation --gcc-toolchain=$(dirname `which gcc`)/../
     This allows relocatability of the toolchain.

I decided to implement the second option and got stuck with implementing it in
cmake. One of the reasons was I was unable to pass --gcc-toolchain early enough
and try_compile fails at very early stages.

Finally I decided to go for the first option hardcoding the gcc toolchain path
at clang built time. This is sufficient for modules.

This commit serializes the progress I did on 2 in configure make. Now we can
configure with --gcc-toolchain in configure-make.

Works for both cmake and classic builds.

ROOT uses builtin gl2ps sources, but has no switch to use the system
library instead. This adds this missing option.

ROOT uses builtin unuran sources, but has no switch to use the system
library instead. This adds this missing option.

The configure script currently uses a simple grep to determine if an option
is deprecated. This causes problems, since the enable-reflex option is
deprecated and grepping for enable-r will result in a match. The configure
script therefore considers enable-r deprecated too, even though it is not.
This commit addresses this issue.

Build with cmake works as it downloads latest Vc.

Signed-off-by: Fons Rademakers <Fons.Rademakers@cern.ch>

Although it is not anymore the base build system, adding tbb support was
not too complicated and allows still easy testing of this new feature.
Use ./configure --enable-tbb. TBB needs to be preinstalled, there is no
support for the on the fly TBB installation featured supported by cmake.

This commit provides a task-based parallelisation of
TTree::GetEntry. The approach is based on creating a task per
top-level branch in order to do the reading, unzipping and
deserialisation in parallel.

Remove the ugly cxxmodule workarounds in graf3d: they don't really work anyway.
Instead, explicitly disable building with cxxmodules graf3d on macos due to
glew. Disable building with cxxmodules libs which still have bugs.

Improve the build system implementation scaffolding for cxxmodules.

Finally, with the introduced infrastructure and workarounds, ROOT builds and
runs with cxxmodules on (given one has newest clang, with my pending for
mainline patch). It seems that roottest is almost happy, too.

Some of the configure code (document as) used for bonjour was also used for XrdUtils

Avoid defining a ROOT_INCLUDE_PATH for R include files and corresponding compilerdata.h file. It is not needed.

Apply patch from Omar to fix inclusion of R header files by defining ROOT_INCLUDE_PATH in thisroot.sh

See documentation at https://root.cern.ch/drupal/content/how-use-r-root-root-r-interface