It cannot be in the Core module because it is included by RConversionRuleParser.h
The RConversionRuleParser.h is used in creation of rootcling_tmp, which in turn
cannot depend on anything (including libCore header files).

This header introduces a layering violation because it is used in libCore and
libThread causing a potential cyclic header dependency. Also, this header is
meant to be included in C translation units, too.

Add an explanation of why this module is kept separate.

Implement the necessary scaffolding and install headers only if IMT is enabled.

This flags allows submodules to be built in insolation from each other.

Eg.
A.h: int i = 4;
B.h: int i() { return 1; }

module M {
  module A { header "A.h" export * }
  module B { header "B.h" export * }
}

Clang would build M in one translation unit, i.e. it will try to compile A.h and
B.h together. This will lead to an error redefining name as different kind of
symbol. If we set the -fmodules-local-module-visibility flag it would work just
fine because the submodules will be parsed and built in isolation.

This patch makes the submodules standalone (by removing the top-most module) on
OSX. This will give us almost the same behavour until the issue is resolved.

AFAIK, the issue is resolved on Apple side but it is waiting to be shipped with
some of the upcoming updates.

This avoids errors such as redefinition of name as diffent symbol when builing
the pyroot module.

This is a temporary workaround on top of a already existing temporary workaround.

This remove duplicate definitions and duplicate default argument errors when
building modules for proof.

Building X3DBuffer.c (in C mode) requires X3DBuffer.h (or the module containing
it) to be able to be built in C mode.

Exporting macros in header files uncontrollably usually hurts. In this case,
Node.inc (from LLVM) tries to define a macro with the same name (BLOCK) but in
a function style way. This causes an error in module builds, we don't see this
in non-module builds because of the 'lucky' alignment of translation units.

This patch works around the bug by undefining the macro before entering the
headers that are redefining it, but we *really* should not expose mmprivate.h in
$ROOTSYS/include because it can alter the behavior of a lot of user code.

TGLIncludes relies on the glew framework, allowing to interact with OpenGL in a
system-independent way. We should always prefer using TGLIncludes.h, and now
we can enforce it by showing an error if we don't to that.

In one module builds we end up in situations in which Quartz or Cocoa include
Cocoa.h or Foundation.h which pulls in gl.h, making the creation of the system
OpenGL module uncontrollable.

Moving GLEW out makes the setup deterministic (we don't rely on special ordering
when building the main ROOT module).

Also, install only the relevant to the current build configuration GLEW headers.

Moving from one header file per module to one module (and submodules) introduces
a few new restrictions. For instance, the module is built if any ot its headers
is included. This means that it will build the TGLIncludes.h-related modules
which in turn requires adjusting the -isystem path on OSX, allowing glew.h
to hooked before #include-ing system's gl.h (building system's OpenGL module).

Our two build systems can generate almost entirely a modulemap file laying out
one module per library. Due to some non-modular implementation in ROOT's core we
still need to treat some header files specially.

This patch appends modular header files to the modulemap while skipping the
predefined ones. Configure-make relates all headers that are installed in
$ROOTSYS/include to their library whereas relates only headers which are part of
library's dictionary.

Core is split differently than the rest of ROOT. We generate the target of
G__Core from within core/base, thus CMAKE_CURRENT_SOURCE_DIR cannot make
headers from the rest of core subfolders relative.

Implement text clipping in TASImage as requested [here](https://sft.its.cern.ch/jira/browse/ROOT-4538).
  Also the text size in batch mode for png (gif jpeg) files better matches the
  size on screen and pdf.

(cherry picked from commit c816e3e4d2ee414215bb899b56ffbc9476553144)

Decouple redirection state from MetaProcessor and MaybeRedirectOutputRAII.
Only suspend redirection when writing to the prompt.
Allow user to redirect to files named 1, 2, &1, &2.
Allow stderr and stdout to be redirected to one another.
Don't create _IO_2_1_stdout_ file.