[core] Generalize TROOT::GetIncludeDir() using build tree marker

[guitargeek]

Introducing a new marker file generalizes TROOT::GetIncludeDir() to
work in all cases for both relocated install and build trees, even if
the CMAKE_INSTALL_INCLUDEDIR is different from include.

This follows up on guitargeek@43ec084, which was incomplete and left ROOT
configuration in a broken state, because ROOTINCDIR was not defined
anymore but still used.

The RConfigure.h header still defines a few other internal macros with
hardcoded paths, and if the suggested mechanism proves to be stable for
the include directory, we can also migrate the other macros. They are
fragile anyway. For example, they contain relative paths for
gnuinstall=OFF and absolute paths for gnuinstall=ON, which is
nowhere documented and breaks relocatable builds. Also, they use
ROOTSYS in the case of gnuinstall=OFF, which we want to migrate away
from.

The ROOT::FoundationUtils::GetIncludeDir() function was also removed
and completely absorbed into TROOT::GetIncludeDir(), as it was not
used in any other place.

Also, allow absolute CMAKE_INSTALL_INCLUDEDIR paths.

Follows up on:

• [core] Avoid ROOTSYS in TSystem include path #20823 (comment)

[guitargeek]

Hi @ellert, I can't think of a better solution. What do you think?

[pcanal]

Could you extend the commit log to explain why the structure is changed to remove ROOT::FoundationUtils::GetIncludeDir (instead of putting the new implementation there).

[guitargeek]

Sure! The explanation is simple: there was not other use of ROOT::FoundationUtils::GetIncludeDir, so the indirection was not needed.

[guitargeek]

Done

[pcanal]

there was not other use of ROOT::FoundationUtils::GetIncludeDir

There used to be ... See the completely related commit 99b31ee.
(ie. Does gDriverConfig->fTROOT__GetIncludeDir() relate to the current work?)

[guitargeek]

For rootcling, the fTROOT_GetIncludeDir() already points to TROOT::GetIncludeDir():
https://github.com/root-project/root/blob/master/core/metacling/src/rootclingTCling.cxx#L40

For rootcling_stage1, there is a separate implementation, which is AFAIK correct and has to only work for the build tree:
https://github.com/root-project/root/blob/master/core/rootcling_stage1/src/rootcling_stage1.cxx#L26

Thanks for pointing out 99b31ee. This commit explains why FoundationUtils::GetIncludeDir() formerly existed but is not relevant anymore.

[github-actions]

Test Results

    22 files      22 suites   3d 20h 9m 22s ⏱️
 3 791 tests  3 791 ✅ 0 💤 0 ❌
80 314 runs  80 314 ✅ 0 💤 0 ❌

Results for commit 78364a5.